Subsequently it became SOP to check both pre and post flight.
Actually a fire system test was always included in the Electrical Before Flight AND After Flight Inspection sheets. We did them too - having a "crew-in" snag on an aircraft on which you'd done the AF/BF was a fast track to the misery of the towing team for the the next three days. A megger could get you airborne off a "crew-in" and once the engine bays warmed up they would usually stay good until the aircraft returned and we could get in to fix it. On a Ranger - well, you already described what can happen away from base...
That Fire-Tech system was a real crock of sh*t which is why its never appeared on another aircraft since the 'V's. Graviner FFFG worked much better and up-dated versions of firewire systems are still fitted in new aeroplanes today. Quite why Avro put crappy Fire-Tech thermocouples around the engines while using FFFG around the fuel tanks, bomb bay and AAPU I'll never understand.