Easy, from the row behind him.
You're supposed to stow your stuff under the seat in front of you.
That's probably why the flight attendant assumed the bag had been left behind - there was no one sitting in the seat behind where the luggage was stashed.
Sometimes there is no space left to put your stowed items, once you get on (if you’re not in Group One, say). You then have to find an overhead space where ever you can...sometimes in rows in front or back of where you’re sitting.
Would it have been so difficult to ask around, if that was a passenger’s bag, before handing it off to ground crew as abandoned/forgotten?