I think you may have been a victim of scale...the housekeeping staff wanted to clean your room ASAP, although I imagine that you'd have things out in the bathroom and other areas in the room that indicated you were still there?
With everyone wanting early checkin and her wanting to cross a room off her list, she jumped the gun

As for disposing of things in the room vs trashing them, thats a tough call...again the scale of these hotels is unreal. The lost and found would have to be a huge space that would allow for everything left in a room, sorted, and accessable. In properties with 4000+ rooms that doesn't seem likely. The places would have so much follow-up they'd probably have to outsource things.

Next time it happens, list off the $2500 tux that you had in that bag for the black tie event you attended and other expensive items that would not normally fit in the safe. I'm sure that MGM is covered to a 'determined amount' for loss/theft/damage...