It's easier if you plant them up the road and out of sight, but generally I find the enemies are oblivious to anything outside the walls until shit happens. Another way to do it is set mines for random passing patrol to get them all stirred up and distracted, get into the main building and shoot them from the windows. Note I have yet to try this in FC4.
EDITED: 23 Jan 2019 13:42 by DSMITHHFX