This is due to the anti-theft system installed in large supermarkets and clothing stores at the entrance. You’ve noticed special gates that emit a specific sound if you bring things from the store to them, right?
Usually, things have a special magnetic clothespin, which is recognized by such gates and notifies the system about the theft of goods if things with such a clothespin are carried through the gate. This clothespin is removed at the checkout when you buy an item.
However, the gate sometimes reacts erroneously to things that belong to you, not the store. Particularly on many bags. This is due to the presence of magnetic fasteners, which manufacturers sometimes use in the design of accessories. This happens not only with bags but also with wallets, for example. And even with magnetic cards to enter your office. If there is a magnet anywhere in your bag, it can be recognized by the anti-theft system. Usually, this error is quickly resolved by the store staff and they apologize for the inconvenience caused.
So, if your bag “squeaked” at the entrance to the supermarket – don’t worry. Usually, store employees are well aware that the system can work with anyone, even if you didn’t steal anything. But don’t worry that security may check the contents of your bag to make sure the system worked in vain. They don’t suspect you. It’s just their job to know for sure.