i found a realy good spot on the barracks:
most people somehow presume you put it on the top, but if you go down UNDER the ramps that lead up, face the outside, and plant it, no one will see it from the outside, they only see it when they're under it and face the outside too
now after doing this, go on the top and stay there, hold on as long as you can, theyll search for a beacon there but wont find it.
i tried this several times and the beacon wasnt disarmed any of the times
somtimes engineers even run past the beacon