To my knowledge no buffer would need to look at the diagrams before being certain. In Dual loop mode both loop detect then signal to MAWEA and bells/red lights.
Also in this case it isnt a 'fault' its a fire....its real.
In case of a failed loop there is a Status mesage set for example
ENG 1 LOOP B.
In my experience these only set when you test the system (this is done on a daily check using the overhead buttons) and find a failed loop.
It should also be noted that the RB211 has a Turbine overheat system linked to the Fire Warning system.