capacitive switch behind a mirror (silver backing)
A mirror typically has a metal coating, protected on one side by glass and on the other by paint.   That will complicate any electrical sensing of a finger.   You might have better luck using a temperature sensor, and having a spot on the mirror sensitive to a warm finger.   Two biased diodes or transistors (base-collector shorted together to make a diode) and a difference sensing amplifier can do the
trick.   On very hot days, you'll have to press your finger to the cold-side
transistor's location, though.