we've started using them at work and the card has to be right on top of the machine's screen to make it beep, also, unless a transaction has been instigated to a pin machine, no card is going to be accidentally triggered, and so if a customer is already at the machine being served, it wouldn't matter who walked past because the customer being served would be using their card and the audible beep only happens when the card is close to the machine, so noone not standing being served need worry about it. It's only 3 transactions under £20 per day allowed anyway.