I've started using "Too localised" for voting to close on questions that don't have enough information, and where the OP hasn't returned.
Is this appropriate? Should I be using the "Low quality" flagging method instead?
I would suggest, rather than "Too Localized", that your close reason would probably fit as "Not A Real Question". If the case is that the question does not contain enough detail, by author absence or otherwise, it is incomplete and cannot be reasonably answered, which falls in line with the description of Not A Real Question.
I might recommend, though, that when using Not A Real Question, that you might consider accompanying it with a comment explaining that it's specifically the incompleteness that is an issue.