I agree that an evacuation is always the safest bet, and I congratulate to the Qantas crew involved (why is it titled as a "drama"?).
Nonetheless I'm not sure if I would evacuate in any case. It's mostly a good move to ask for information from people outside the flight deck, i.e. CAs and tower. If there is a real fire, then there are obvious signs of it. If the fire is so small that you cannot see anything, then there is most probably enough time for the fire brigade to have a look inside the cargo bay.
I also have to disagree with egbt's statements: a) these sensors are not fire detections (like they are e.g. in the engine nacelles), but smoke detection systems. b) yes, they are relatively reliable, most of all in a controlled environment like the developement room - in the outside world it happens from time to time that you receive false warnings. If I evacuated everytime I had a smoke warning, I would have performed quite a lot of them... I'm talking here mostly about detection systems in smaller airliners, not those in Airbus and Boeings.
Now start shooting...