It's been awhile ago now, but there was a time I was leaving a Walmart after checking out, and a person timed their fast exit through the alarm deal just as I was passing through setting it off. They looked through my stuff.
Also, for about a year everytime my mother would walk through one of those the alarm would go off whether entering or exiting. Somehow only was searched once. That pissed her off. A bit later she was entering a small store once and set it off. An employee checked her purse and realized that it still had the security tag in it. She removed it for her. This was a sold 20 years ago.