The unloading areas are within the attraction so they are secure. We were on Everest this summer and a gentleman with great arm strength flipped himself onto the ride and when it was over the chair was waiting exactly where he was to disembark in the unload area. He flipped himself right into it and was gone before we could even gather our belongings. Amazing. So yes they are secure. Also, I don't think a guest would get very far with it. Security is good, and if you immediately told any CM it was gone all CM eyes would be peeled looking for one of that description. Disney would not look kindly on that kind of a theft. My DD8 at the time was in a wheelchair for one visit due to an injury to her knee. All the CM's were so kind to her.
Some people are like Slinkies.
They aren't really good for anything,
but they still bring a smile to my face when I push them down a flight of stairs.
Friends are Gods way of apologizing to us for our families.