No, nothing ever stolen
But, while waiting for DH, DD and DS1 to come out of a ride I have seem several times a family start to load a baby into a stroller and then realize it's not theirs ! And then un-load, find their stroller and re-load
I have seen before one person load up a stroller, and have the correct owner come to get theirs and find somebody took it by mistake. I generally think this happens with strollers that look alike.
There was only 1 time I heard or saw and employee fuss at people about leaving things in the stroller. It was at Stiches Great Escape. She was announcing to everybody, make sure everything was actually IN the stroller. No bags hanging on the handles, etc. She said bags handing on the handles are considered unattended bags and would be collected. ( what-eva) Our diaper bag is the style that has the stoller handle hooks and was hooked on our handle. She made it a point to put it in the basket. But she has been the one and only time I've ever heard an attendant make any fuss.