I dunno - the fine upstanding citizen in me says to ask - the financially savvy part of me says to do nothing so you don't upset the balance.
Because the entire building is in the same situation as yourself, I don't think that it is an oversight, I imagine she is just an extraordinarily nice woman.
However, keep in mind that you still may be liable, so I would continue to deposit the money in your savings account until you eventually (though hopefully never
) move.
If there isn't a signed contract, in the eyes of the law (at least here in Wisconsin) you are under a verbal month to month lease. Basically, either party can leave with as long as they give 30 days notice.