The two grey slabs you walk between on the way out of a shop are not scanning you, your bag or your shopping. They are doing something far simpler and far dumber. One of them is broadcasting a signal into the gap, the other is listening for a specific echo, and if it hears that echo it makes a noise. That is the entire intelligence of the system. The gates cannot tell what you are carrying, cannot tell whether you paid for it, and cannot tell the difference between a stolen coat and a jar of face cream you bought in a different shop half an hour ago.
Which is exactly why they beep at innocent people so often. The pedestals are listening for one thing: an active security tag, of a type they recognise, passing between them. The tag on the coat you stole and the label on the moisturiser that was not properly switched off at the till in Boots produce the same echo. The gate has no way of knowing which is which, and no shop has ever wanted to pay for one that could. Here is what is actually going on inside those pillars.
Two technologies, one job
The technology is called electronic article surveillance, or EAS, and it has been around since the 1960s. Almost every British shop uses one of two flavours.
Radio frequency, or RF. One pedestal sweeps a signal back and forth across a narrow band centred on 8.2 megahertz, about sixty times a second. A tag contains a coil of wire and a capacitor, a component that stores a small charge, tuned to resonate at that frequency. When the sweep hits the tag’s resonant point, the tag sucks a tiny amount of energy out of the field and dumps it back, and the receiving pedestal sees a dip and a spike at exactly the frequency it was expecting. Cheap to make, cheap to install, and dominant in fashion.
Acousto-magnetic, or AM. This one is properly clever. The tag holds a strip of alloy that physically vibrates in a magnetic field, tuned by a second magnetised strip beside it. The pedestal transmits a short pulse at 58 kilohertz, then stops and listens. The strip rings on for a few milliseconds after the pulse ends, exactly like a struck tuning fork, and the receiver hears it in the silence. Because it listens in the quiet gap rather than through its own transmission, AM shrugs off interference that would swamp an RF system, which is why supermarkets favour it.
A third type, electromagnetic strips, survives mainly in libraries and archives because the tags are thin enough to hide in a book spine and can be switched on and off endlessly. You will meet those again if you have ever set off the gate at a library while carrying a book you did borrow.
The invisible geometry
The detection zone is not the doorway. It is a bubble between and above the pedestals, strongest in the middle at waist height and weakest at floor level and around the outer edges. This is not an accident, it is the shape a magnetic field naturally takes between two coils, and it drives a lot of shop design.
RF systems reliably cover an aisle of about a metre and a half. AM systems stretch to well over two metres, which is why a supermarket with a wide exit tends to have AM. Every extra centimetre of aisle costs detection certainty, so the width of the gap you walk through is a decision somebody made with a spreadsheet, balancing wheelchair access and trolley width against the number of coats that will walk out.
It also explains why shops keep fitting floor level antennas and narrowing their exits, rather than buying cleverer detection. The weak spots in the field are geometric, so the fix is geometric too.
Why you beep three shops later
Hard plastic tags come off at the till. Soft labels, the flat white sticky ones on packaging, do not. They are deactivated: the till has a pad that blasts the label with a field strong enough to permanently detune it, breaking the resonance so it can no longer answer.
Deactivation is a physical process and it is not always complete. A label held slightly too far from the pad, or shielded by a metal tin, or at the wrong angle, can end up half killed: too weak for the shop’s own gates but still just live enough to trip a more sensitive system elsewhere. Since almost every RF shop in the country tunes to the same 8.2 megahertz, your Sainsbury’s label works perfectly well as an alarm trigger in Boots.
Then there is the innocent physics. Certain metal objects, some cosmetic and foil packaging, occasional medical devices and mobility aids all happen to resonate near enough to the right frequency to be mistaken for a tag. Nobody is doing anything wrong. The universe is simply full of accidental tuning forks.
The foil bag arms race
The best known countermeasure is a bag lined with metal foil, which works for exactly the reason a microwave door has a mesh in it. A continuous conductive layer forms a Faraday cage: radio energy cannot get in to excite the tag, and the tag’s response cannot get out. The tag is not disabled, it is simply muffled.
Retailers responded in two ways. Technically, modern pedestals often include metal detection alongside EAS, so a large sheet of foil moving through the gate raises its own quiet alert on the staff console even though the shopper’s tag never spoke. Legally, carrying a purpose made shielded bag is treated as evidence of intent, and going equipped for theft is an offence in its own right under the Theft Act 1968 carrying up to three years. That combination has made the tactic considerably less attractive than the internet suggests.
False alarms are a setting, not a fault
Here is the part that surprises people. Retailers could turn the sensitivity up. They deliberately do not.
Every EAS system has a dial, and the dial trades missed detections against false ones. Wound up high, you catch more thieves and also stop a customer every few minutes, most of them innocent, each stop costing staff time and goodwill. Wound down, you miss more thefts but the shop floor stays calm.
Almost every British retailer sits well below maximum, accepting a known miss rate as the price of not accusing customers all day. So when the gate goes off behind you and staff wave you on with barely a glance, they are not being lax. They have been told, correctly, that the overwhelming majority of alarms are nothing.
The commercial pressure behind that dial is real, mind you. The British Retail Consortium counted 5.5 million detected shoplifting incidents in its most recent survey, costing retailers close to £400 million, and police in England and Wales recorded 519,516 shoplifting offences in the year to June 2026, down 2% on the year before after a period of increases, according to the Office for National Statistics. Retailers have spent more than £5 billion on security over the last five years, the BRC says. The gates stay switched on. They just stay switched on politely.
What happens if it goes off and you have done nothing
Stop, and stay relaxed. An alarm is not evidence of anything and staff know it. You will be asked to show a receipt or walk back through, which is usually enough to identify a stray label.
Shop staff have no general power to search you or your bag without consent. They can ask, and you can decline, though declining tends to prolong the conversation. Detention is only lawful in narrow circumstances where someone is reasonably suspected of an indictable offence, and a beeping gate does not get anyone there. Most of the time it lasts fifteen seconds and ends with a live label on a packet of razor blades bought at lunchtime.
What is replacing them
The old pedestals are slowly being outclassed by tags that know what they are. An RFID label, short for radio frequency identification, carries a unique serial number for that specific item, so a doorway reader can announce not merely that something left but that a size 12 navy jacket left, at 14:52, and that it was never sold. That turns a noise into a record, which is a different tool entirely.
Retailers including Marks and Spencer, Zara, Decathlon and Primark already tag garments this way for stock counting, and using the same tags for the door is nearly free once they are there. Tesco’s own long journey with the technology is worth reading in our piece on how Tesco went from sceptic to advocate. If you want to know what is inside the plastic lump itself, we took one apart in what the hard plastic tag on clothes actually does.
Frequently asked questions
How do shop security gates actually work?
One pedestal transmits and the other listens for a response from a tuned tag. Radio frequency systems sweep around 8.2 megahertz and listen for a resonance. Acousto-magnetic systems send a pulse at 58 kilohertz and listen for the tag still ringing in the silence afterwards.
Why did the alarm go off when I had not stolen anything?
Usually a label from an earlier purchase that was not fully deactivated at the till, since most British shops use the same frequency. Occasionally an object that happens to resonate near it, such as certain foil packaging or metal items.
Can the gates tell what I am carrying?
No. A standard EAS gate detects that a live tag passed through and nothing more. It cannot identify the item, the shop it came from or whether it was paid for. Newer RFID based systems can identify the exact item, but they are not yet the norm.
Do foil lined bags really defeat shop alarms?
A continuous metal layer does block the signal, which is why the technique exists. Many modern pedestals detect large metal masses separately, and carrying equipment made for the purpose is an offence in itself.
Can shop staff search my bag if the alarm sounds?
They can ask, and many people agree, but they have no general power to search you without consent. An alarm alone is not grounds for detention. Showing a receipt almost always ends it.
Why do shops not just make the gates more sensitive?
Because sensitivity trades missed thefts against false alarms, and a shop that stops an innocent customer every few minutes loses more in goodwill and staff time than it saves in stock. The setting is a commercial judgement, not a technical limit.
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