For RA2 it was an add-on official map (rockets.mmx)
Which is why I have some memory of the triggers, as I always examined the maps when they came out
Anyway my recollection was faulty, it uses #47 Elapsed Scenario Time (isn't that odd). I'd swear there was a map that used 'discovered by player', and that was to play ambient water sounds at the foot of a cliff. Oh well.