Reasonable suspicion is the sirens going off from the magnetic strip on some packages. First time it happened, I did stop, but raised hell with the manager because of the incompetence of the cashier that did not deactivate it.
Another time, the cashier was not able to get to it on a large item so she said she'd just wave me on at the door. What she did not know was the device came of easily and I stuck it on the bottom of the cart for the next user.