Completely invivisble or cloaked kind of invisible?
There are two methods:
The method they most likely used was the trigger 'Set Doodad State' to disabled, although that removes several commands you can do sometimes, and usually crashes.
The other way is mentioned
hereCompletely invisible is done by doing the above, then either creating an observer or other detector above it for an allied vision player, and then removing it, OR giving the building to an allied vision player and then giving it back to a player without allied vision.
The building must be owned by a computer player, or (posibbly) disabled vision to yourself.