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.
That is a fair point, however, in that case, there would be a computer left over on the belt, that the author would've seen when he got out of the scanner.
On a side note: do people not keep their laptops in sleeves (even when in backpacks)?
I'm surprised that everyone is focusing on the TSA for giving his computer away (awful), but to me, the even more objectionable thing is that THE WOMAN TOOK A RANDOM COMPUTER and just left with it. Any reasonable person would say "that is not mine", or am I being too naive? And, since she didn't do that, there was clearly malice involved, so the police were also clearly in the wrong, saying that it was an accident and not a crime.
I am amused how the article states that the iPhone was an "instant hit". It was successful-ish for the time - it sold around 6million in 5 quarters - but was also being scoffed at by every other dumb- or smartphone maker. Same with the iPod I guess... and iPad... and everything that has evolved and changed their respective markets in their iterations.