KB: I fully agree that you need to be vigilant. I try to stay with my case and not let it go into the x-ray until I am next in line for my person to be scanned. And I try to keep my eyes on the x-ray machine as I go through my personal scan.
So far, I have kept all my things. But as your cabin crew suggested, not-so-petty theft happens. I'm sure my surveillance actions also slow down the line a bit, but I am going to still keep doing them. The point of my original post was simply that the idea of a property inventory and receipt both going in and coming out of the x-ray is unworkable.
Note that theft also happens from checked baggage, too. We can only try to minimize baggage or security line theft by vigilance, avoiding traveling with stuff that is real expensive, and have insurance.