At Lyneham in the early 70's, and "AIR RAID WARNING RED" came over the Tannoy so all personnel were ordered to take cover in their slit trenches.
Said trenches did not actually exist, but were realistically simulated by white tapes on the grass. Shortly afterwards a bemused TACEVAL umpire enquired of a bunch of tired, cold and generally pi**ed off techies why they were standing on the volley-ball court.
On the same occasion, a couple of Lightnings were sent over to make lots of noise to add 'realism' to the scene by simulating an enemy air raid. Unfortunately, the weather was so c**p that they had to request a GCA appraoch to locate the target!