I went to Costco tonight and picked up about a dozen things. I found a line which looked suspiciously shorter than the others - I'm always a sucker for those lines, even though it seems that many times they're slower than the other ones. At any rate, the guy before me was almost ready to start loading his stuff onto the belt when something happened that I had never seen before: I got pre-scanned.
A woman approached me with a scanning gun and scanned my card and the items in the cart. Once that was done, I just proceeded to the register without taking my items out of the cart. Once the checker scanned my card, everything showed up on the register, and he had the credit card receipt ready for me to sign before I could even get down to that part of the lane. Very fast, and arguably, very un-Costco-like.
Once home, I started searching around to see how many other people had seen pre-scanning, and I found references back for at least a few years. Everyone seems to really like it, but I can imagine people behind you who didn't see you get pre-scanned must wonder what in the world is happening.
While pre-scanning seems to speed things up, I can also see how the people doing it have to be selective about which customers they target. It probably wouldn't make sense to pre-scan people who only have a couple items, since it's probably just as fast for the checker to do the scanning. Also, if you have your cart packed, it'd be more trouble to get to all the items than it's worth.
So now I know to only get enough stuff to fit on about one layer on the bottom of the cart and look for a person wielding a scanning gun.