Earlier quoted context omitted.
It's really tunable and some stores are tuned to "Fort Knox" where even a fly landing UNKNOWN ITEM IN BAGGING AREA. Others are so loose that you don't even have to take things out of your cart, scan and pay and go.
The Whole Foods location nearest to me has self-checkout stations that don't even have a scale under the bagging area. You just scan, put it in your bag and go. I assume they can afford to tank the shrinkage that results from this due to their high profit margins, or they just don't consider the costs associated with the scales to be worth it.
It uses your rewards card to determine your “risk”, I think. Just a theory. But whenever I’ve gotten a new card, they come check my bag a lot. Then after a dozen successes or so, they stop checking so often. Then hardly at all.
One of my friends forgot to scan an item once when they checked, and he had someone come to check his bag way more often for a while.