Your employee is sending you a message, but ironically, it is difficult to determine what she is trying to say. Since this is one isolated incident, it is unfair to draw any conclusions. However, one can draw some hypotheses.
On the surface, this type of comment in front of your manager can easily be seen as a way for your employee to undercut your role and relevance while placing herself in the limelight. At the same time, it is possible that your employee harmlessly used this expression simply as a way to help communicate a point, and she had no sinister motives in doing so. The best way to understand her real message is to look at her overall behavior. Do you see these words as a part of a pattern, or was this an isolated incident? Your answer to that question will help you answer your larger question.
Either way, since you are upset with her comment, you should tell her what you think about her comment, and make sure she knows exactly what you are trying to say.
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