To be fair, MacBook Pros pretty much all look the same on the outside. She probabaly just thought it was hers, stuffed it in her bag, and went to catch her flight.
Hey, a name game: MacRoulette. You put your laptop through security with a bunch of others and you grab a random one. Practice safe computing and always have a backup.
On a side note: do people not keep their laptops in sleeves (even when in backpacks)?