Once we left a bag on a bus when we were returning to the Fort. We have 3 kids and double stroller we lug around...so each of us thought the other person had it.
Our bag went back to the park we came from (Hollywood Studios). There is a lost and found at each park. If the item is not claimed there, they told us that it travels to a central location for all lost and found items.
After a few hours...we finally received a call from the "manager" of lost and found. They asked us to describe the bag (it was one of the canvas bags that you get for $10 with a $40 purchase

). It had a Nikon digital SLR camera in it along with a ton of kid's stuff. They brought it to our campsite about 15 minutes later!