If something is in plain sight or a logical spot like the drawers, the housekeeping staff will usually find it and bring it to lost & found for the resort. But if something is in a slightly different spot, sometimes it can be missed by housekeeping, and it would be up to the next guest in that room - if they are ethical, they'll report it and turn it over to staff - but if it's something valuable or interesting that they like, and they lack ethics and morals, they may just decide to keep it...then there's not much chance you'll get it back.
In all my stays at Disney, I've found items in the room 3 times - it doesn't happen often. Usually it's when they're stored somewhere like the coffee-table storage units that open up, or in the laundry machine...or behind the end table in the bedroom (the three places I've found items). I notified the front desk of the items and turned them over - the dryer items were just a few clothing items, and the items in the coffee table storage were Minnie ears and a child's tutu...the third item was a very nice jewelry gift from the stores in a bag with a 3-figure value - the first two most people would likely report or turn over, but some less ethical folks might have kept item #3...that's where luck comes into play in deciding whether you'll get your item back.