Our Maclaren stroller was stolen during our first trip from the MK. We did hear from the many helpful CMs that it is a rare occurance but does sometimes happen. We were lucky that no other personal objects were in the stroller besides a picture of us from the Judge's Tent that was in the stroller bottom. The CMs got a rental stroller for us (for free) and brought it right to where we were. They gave us free stroller rental vouchers for the remainder of our trip and checked many times at lost and found for ours, just in case. It was never turned in as far as we know and we called there and checked often too.
Like I said, it is rare, but I wouldn't leave anything of value in it while you ride, just in case!