I once forgot a bag of duty free in the overhead bin on a Thai Air flight landing at JFK. While waiting at the luggage carousel there was an announcement that a bag had been left in the bin over seats XX A&B, and I embarrassedly retrieved it from the smiling Thai FA.

I like to think that most people - passengers or airline/cleaning crew - are honest, but some are not. Sometimes forgotten items are turned into lost and found, sometimes it's "finders, keepers." I was lucky. It appears that the OP was not.