It started a few months ago where every now and again the alarm would go off when I entered and exited the store. Not every store and not every time. I definitely remember it happening in Asda, Lidl and B&Q.
After a while I realised it was the credit and debit cards in my wallet that were setting the alarms off.
I have quite a collection (mainly from unused credit card accounts where I have shifted the balance around) and it seems that if you stack enough contactless cards together the signal is strong enough to trigger the security alarm!
It first dawned on me what was happening as I was leaving B&Q one time. I tested it by taking out my wallet and holding it up to the barrier thing and sure enough the alarm triggered. I took out a bunch of cards and held them up one by one and nothing, no alarm. But when I grouped a bunch of them together that's when the alarm went off.
So there you go. Contactless bank cards are capable of setting off security alarms.
You may have already been aware of that but it was a revelation to me, and a relief because it did cause one or two awkward moments where I had to convince the security guard I wasn't a tea leaf!