Right, an evacuation would NOT take place without the alarm going off. A procedure is to evacuate using public address followed by activation of the alarm. The alarm horn is cancelled by the crew to prevent panic and so shouted commands can be heard. The 747 cabin is large. The crew are by the doors. It is not inconceivable most people simply couldnīt hear it.
Once outside, what was the danger? It appears the problem was caused by acrid smoke, probably within the aircon system. I have had such an incident myself, which was kept low grade, evac by the door and up the jetty. The situation was being monitored internally and externally. There was less risk keeping people nearby than getting them spread out over all the apron and lost. Unless visible signs of fire are present, there is no risk. Had any flames appeared, people would have moved away. It was contained. It seems to me, the situation was extremely well handled. You do not initiate a full evacuation lightly- it must be carefully controlled from the flight deck relying on all sources of information, internal and external.
Once again , we have a well handled situation being sniped at by instant self appointed 'experts' here. Right now a very comprehensive review is being undertaken by BA management who are experts in this sort of event. They will cover all aspects. By all means ask, but donīt come out with self dreamt up theories and criticisms on why things happened as they did. I have been through all this training which was executed here. They did what they did well.