Another Hypothetical:
During the preflight procedure the Flight Crew Notice the "CARGO HEAT ON" light is NOT illuminated preflight with the switch in the ON position.
Flight Crew conduct a Annunciator Light Test & the light does not illuminate.
With the absence of any EICAS massage indicating a fault with the cargo Heat System the aircraft departs with the reasoning the bulb is failed & it will be written up and changed on return to a main port.