Halliday & Matthiessen (2014: 128):
In this type of ellipsis the clause is not presupposing anything from what has gone before, but simply taking advantage of the rhetorical structure of the situation, specifically the roles of speaker and listener. Hence the Subject, and often also the finite verb, is ‘understood’ from the context; e.g. Thirsty? (‘are you thirsty?’), No idea. (‘I’ve no idea’), A song! (‘let’s have a song!’), Feeling better? (‘are you feeling better?’). Such clauses have, in fact, a thematic structure, but it consists of Rheme only. The Theme is (part of) what is omitted in the ellipsis.