How is this "colleague" (who's no colleague of mine) able to check whether I've been overcharged? What they do is rootle around in my bags, scanning random items. They can't detect whether I've been overcharged or not: their scanner uses the same pricing system as the customers' scanners. They could perhaps check that I'd not scanned the same item twice, but they'd have to rescan everything in my bags to do that, not just a random sample.
Just tell it like it is, Sainsbury's: you want to check that I haven't shoplifted anything. We all know that's why you're doing it, and pretending you're doing it for different reasons just makes you look insincere.