From the looks of it... it sounds like you overloaded the object.
Ex: Everyone gets ontop of a vehicle. Eventually the vehicle will disappear. Like in Halo 2 on Zanzibar. If you get enough people to stand ontop of the bridge to the sword it will overload and become invisible.
This only a theory fo course. I suppose you could try it out. It also could have been what twiggy said but ive never seen it happen.