The closest I have come to the ground situation here was on an AA flight at the gate preparing to depart. What appeared to be smoke started to descend from the ceiling vents and there was a smell like burning hair. The cabin crew were nowhere to be seen and there was no announcement. Everyone remained seated and calm. Eventually someone got up and went in search of the crew. He came back to report that they were huddled in the front galley with damp cloths over their noses. One of them took the cloth off long enough to tell him that it was just that de-icing fluid had got into the ventilation system and it was perfectly safe. That was certainly a case where there was no reason not to make an announcement yet none was made. It is a good thing that passengers are willing to remain calm and await instructions.