These guys can be a big annoyance. Gone to the one builders once where the tills are position such that the Slip check guy can see everything being rung up and placed in the packets. I've gone to the till right in front of him, items being scanned while he watches and then ask to see the slip and inside the bags, I just said no. You just watched everything being scanned and packaged.
A bit of an irritation and most of the time they don't really check anything, just mark and stamp