My significant other owned a trailer at one point some time ago, and when I informed him in passing that Holly's trailer has been demolished he was shocked...I'm quoting what he said:
There is actually another floor *under* the main floor. The floor that people walk on is a sub-floor. There is an open space in most older trailers between the floor of the trailer and the subfloor. The space is large enough in some cases that a person could fit in there.
Like Tracker said, there are many hiding spots. There are cubby hole like areas in the walls.
There is a space of several inches between the outside wall and the inside wall, which is probably drywall. The insulation goes in here. It may not have looked accessed from the inside, but if there was siding on the trailer, someone could have easily removed a piece of siding from the outside, put something inside the wall, then replaced the siding.
It was confusing to him why the owner paid the extra expense to have trailers demo'd on site. It would be much cheaper to have them removed to an outside location to be demo'd. There would have been lots of extra clean up costs. Also, usually if a trailer is being demo'd, it gets stripped and scrapped. Pipes, copper, metals, wires, etc all bring the owner a profit. This is what was stunning to my S.O. That scrap money was tossed away, and the extra expense of hiring a demo crew to come on site and also do cleanup.