Feedback but no examples from boss

What's the best way/s to respond when your boss provides feedback (not MT style but just feedback in general), but she can't think of an example to illustrate that feedback, though it would be really helpful to have? Otherwise I'm scratching my head a bit about how to address it / what to do going forward.

Of course, I can be mindful of the feedback and try to be more attuned to anything that might have prompted it. But is it impolitic to bring it back up and see if she can provide any examples (realizing the manner in which it is brought it would be key)?