Very interesting.

Things won't always work this way, of course. The authorities may just tell you what, not why. Since you don't know why, you might attempt to evacuate right into more trouble. But I think in general, it's safe to make a more conservative decision than the authorities. For example, building security says "stay put". Well, of course they do, not only is the information incomplete and slow to arrive, but they have tenants who'll be upset if all their employees are told to leave without a really good reason. Me? I'll take the chance... I'd rather feel silly on the ground than die in the building.