Well, I had a smuggling compartment in a garage (a couple of houses ago).
It was a split entry house - to enter the house, you walk up a few steps, through the door, and on to a landing - you can then go either up or down.
The stairwell / landing was right next to the garage. There was no access to the area under the landing (but one closet under the stairwell). I got to thinking... What's under the landing?
So I cut a hole in the sheetrock in the garage. Whoa. There's an empty space under there... 8 feet by 8 feet by about 3 feet high.
Next thing you know, I had it full of wheels, seats, mufflers, boxes, and anything else I could stuff in there.

I cut the sheetrock so that I could simply slide it back in to place. Yeah, the cut was "kinda obvious", but the ~200 cubic feet of storage space more than made up for it.