I worked at a grocery store for 3 years. The policy was you immediately dropped your bag off in a locker in the break room when you got to the store, and you didn't pick it up until you left. You were never able to have personal bags with you while you were by the merchandise, so there was no way for merchandise to end up in your bag.
This seems like an incredibly obvious solution to me - what reason would an employee have to have their personal bag on them in the storeroom or on the floor? The only time you might want your bag / anything in it is during a break, right?