Are you talking about a normal mirror or a smart mirror? Lol
If it is a smart mirror, and it was the region where your screen or RPi is located, that will be because of the heat they generate. It’s like how heated windows and side view mirrors on a car work.
A trick to prevent any bathroom mirror from fogging up as you shower is to generously smear shaving cream (not gel) on it and spread it around nicely, and then wipe it off with a cloth after 5 mins. Prevents it from fogging up during hot showers.