The most frightening engine fire warning I had was just after lift off after a rotors running refuel, where some fuel had overflowed, when it could be argued that a fire is more likely than at other times. It also the first sortie of the years after a long Christmas break; just what you don't need on your first day back.
I landed on with both engines running, shut both down then rapidly diagnosed that there was no actual fire at all, so we also saved the fire bottle (we only had one bottle between two engines, typical MOD cost-cutting
). The nut and washers securing the wiring to an engine bay fire detector (bi-metallic strip type) had come off. The wiring had dropped down and earthed out, putting on the FIRE light.