So anyone who gets their trolley checked to make sure they haven't been over or under-charged is a muppet?
Fine logic there, well done. :roll:
They are not checking to see if you have been undercharged, only to check that you are not colluding with their dishonest staff to steal.
Costco must have real concerns about the dishonesty of their staff, because it is usual to have two members of staff working each till. One directing the customer and helping empty the trolley (or for large items that are left in, putting them in a position to be scanned). Higher value items are all empty boxes and are brought from the security store when paid for.
So to get something through the tills in the trolley without payment is very unlikely without the collaboration of at least one dishonest member of Costco staff.
At the security check all they do is count the number of items in the trolley to check the number is not higher than the number of items on the receipt. They don't check if it is lower (I know, I checked).
Now it is bad enough that Costco distrusts its customers and staff so much that it puts in place this exit check, but then it doesn't staff it properly.
So you have a long queue of muppets waiting to have their trolley checked for goods that they had just paid for, because Costco believes it has employed a higher number of dishonest staff than other retailers.