I got checked a 2-3 times when I first started using self scan and never missed anything and now I cannot remember the last time I was check. Maybe you build up some trust within the system?
It depends on the system and how it is implemented but there are various types of trust system or it can be as basic as check every X baskets per Y shopping trips across all terminals. For the high value items I think it less about that and more about basket value compared to how much your basket value normally is. not 100% certain on though.If you're getting randomly checked often (scan as you shop), then you're either adding and removing items from the list of scanned products or you're triggering one of the other criteria for a search like adding high value items that you don't normally buy.
Try it; add a bottle of ketchup then remove it from the scanned list. Then scan a different bottle... you'll end up getting searched.
I like scan as you shop but I tend to only scan the item after I'm 100% sure I won't need to remove it, to avoid the faff.
Its meant to deter theft as it gives you the impression you are being recorded.The only issue I have with self serve tills is the ones we have in the local coop, they have cameras on the top, wtf for, everytime I go it wants to wait for a member of staff who is either walking around stacking shelves and the likes or serving people on the main tills to say I'm old enough to buy beer, surely without much effort they could have the system ID you from a picture its taken and then use that to determine age, or, given every time I go in I get verified by staff, surely putting the two together isn't that hard...