1. They select an item... 2. They conceal the item...
Once this occurs, you must maintain visual contact with that individual 100% of the time until they leave. If you don't, you could not stop them (they may have "dropped" it on another shelf when they were out of site...)...
Arizona may be the only state that has a specific law against concealing an item inside a store, then leaving without it. It was passed during a fad for the practice you descdroibe a few years ago.
Tehcnically, I suppose you COULD here, but people do strange things... sometimes, they'd be shopping, and get a can of chew and at some point, out of habit, put it in their back pocket (ever se those "chew can rings" on the back pockets of jeans?) and finish shopping... I[d just go to the checkout when they get there, and quietly (so as not to cause a scene) say to them, "don't forget to ring up the can of chew" and you could see their embarrassment at making such an innocent, yet potentially expensive mistake... they almost always thanked me for it...
After a while, you could tell who was a thief, and who just made a mistake...