So why are they stealing laptops in an airport? They wouldn't: high risk, low reward.
The way I see it, it was simply an error on the part of the TSA, as discussed countless times in other comments.
Or maybe she didn't realize for a while and by that point, a combination of laziness and desire to keep this shiny, luxury model computer that had accidentally ended up in her possession led her to just hold onto it.
I think the number of people who will decide to keep something they find (dropped wallet, misplaced laptop, etc) is much higher than the number who would actively go out of their way to steal something. The end result is the same but I think psychologically, it takes effort to go out and steal whereas it takes effort to track down an owner while keeping a found item just requires apathy and/or greed.
Every time I've ever flown, my boarding pass is marked up by a TSA agent before I go through the metal detector/body scanner/X-ray belt area. I think it'd be incredibly suspicious if you left and then back through, multiple times even more so.
Once we figured out that it wasn't actually difficult, we repeated once more before we left. Horrible habit I know, but on long flights/connections you spend near on a whole day "airside". It's pretty simple to just explain why you wanted to nip outside for some fresh* air.