I've been trying for some time to put my finger on what makes us choose, in everyday speech, between Где моя сумка? vs. А где моя сумка?, or Мы квартиру покупаем vs. А мы квартиру покупаем.
I'm not considering those cases where а clearly has its ordinary contrastive function (Ты тут недавно! — А ты, что ли, давно?), or where it strings together a narrative of back-and-forth action (...А он этот бриллиант спрятал. А они нашли. А потом оказалось, что он вообще фальшивый.)
My curiosity concerns just the "out of the blue" kind of а which seems to have no roots in a conversation's context — and indeed often seems to break that context. The best I could come up with was that this а was a sort of call to attention to the whole uttering as something new and important.
Is there a better explanation? Has it been described as a more general linguistics phenomenon (because it has clear similarities to, as well as clear differences from, e.g. the Semitic sentence-initial "and")?