There have been stories about Costco exit check not being about theft at all, it’s mostly about checking for possible errors or fraud on the part of the cashier
Back when Costco accepted Discover and allowed cash-over on purchases, when I took cash over, I would be asked by the receipt-checker, "did you get your $30?". On two separate occasions, I had in fact forgotten. One of those, I was given the missing cash immediately without question. The other time, the manager was skeptical but agreed to close and count the drawer, and I received the $30 immediately after that process.
I've had them catch mistakes before, like an item being double-scanned. I was actually pretty impressed that they looked at the total count on the receipt and cross-checked it against the contents of the cart.