Again, not a computer techie, so this is probably redundant but; Could you set it so there is an AI behind/on the mirror, that has a 180 degree arc of vision, so when the player moves into enough light to make them visible, the mirror AI will see them and make a sound only 'actual' AI can hear. (That's a long winded, poorly written sentance...) It has flaws, but if it were possible, it would be another method.