I work in a retail store that has those scanners and they go off all the time—many times when no one is even near them!
I will also tell you that Thom Mcan shoes set them off in other stores even if they are deactivated at the purchasing store. Something about the magnet they use...
So, no, the devices are not flawless, and at my company we have been told not to stop or pursue anyone who refuses to come back when they go off.
At some establishments those alarms do seem to have a lot of false negatives. Generally a short glance back at the staff will let you know if you need to stop or not. Generally they just wave you on.