Departed Singapore Northbound with a 747 freighter, just past Kuala Lumpur got a main deck fire warning, but no sign of smoke or distress observed having sent a spare crew member down to the cargo deck with a fire extinguisher. Turned back towards Kuala Lumpur but then realised that we had to spend time dumping fuel down to landing weight anyway, so continued back to Singapore and started dumping. As we landed the fire warning light went out.
Previous flight had been full of Durians, the heavy, cloying vapour had fooled the sensors, but of course we didn't know that.
But I agree, one doesn't always know, and each case is different, he could have held over ANC to get down to landing weight if that was a problem, but one is never really very far from a "suitable " airport twixt LAX and ANC.