He forgot his laptop, because it wasn't on the belt when he came out of the scanner. They gave it away before he had a chance to forget it. The couple of minutes it took him to realise that he didn't actually all of his stuff off the belt (even though, he did, since the laptop wasn't there any more) wouldn't have made a difference since somebody else has taken it beforehand anyway.

Especially if you're used to being able to leave your laptop in your bag - you fall back to routine very easily. I want to fault him for walking off and leaving it, but I can also imagine myself easily doing the same.