It’s up to the DM.
It’s up to the DM to decide what the player experiences, and to then relate that experience to the rules for perceiving magical effects. In this case, we have this rule concerning spells from the Player’s Handbook:
Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed, unless a spell says otherwise.
Obviously this is talking about spells, but it should be obvious that it applies to other magical effects. So it is up to the DM to decide what a player experiences when they look at the mirror. The item description does not say.