Here's an odd one:
I started setting off the alarms in Sainsbury's. The first time, I went to the loo and it went off as I and two others walked in. When I left, on my own, it went off again! Puzzled, I left the store to return to my wife and car. The exit alarm went off too - but nobody took any notice, so I shrugged my shoulders and carried on.
A few days later, I set off the alarms again in a different branch of Sainsbury's. This time, I was stopped by security.
It wasn't the shopping - only me!
I went through the contents of my pockets until I got to my wallet, which turned out to be the culprit. "Oh!", I said, "It must be objecting to my Tesco loyalty card!". The guy seemed happy, so I left.
A couple of weeks later, I was sorting out all the receipts and credit/debit card slips in my wallet when I came across an unused (protective backing still in place) RF ID tag, which was obviously the problem.
So, did the cashier think it would be fun to slip one of these in with the receipt and other bumpf I was handed - presumably at the first store where I had a problem?
I can't think of any other explanation ...