It may just be a semantical wording/naming issue. Confirm with your boss that the bonus is for "time served" and not for a future commitment. If you have a really good relationship with your boss you can simply say (with a smile), "So I can leave next week and still get to keep the money?"

If it is in fact for time served then it's really just a regular performance bonus, that it is specifically because you stuck around is making someone call it "retention" but, based on your description, it's tied to past performance rather than future performance.