.. Glowing mirror in the bathroom

Look at it. There's a blurry symbol. You need to make the symbol visible.

Check out the fauce. Try to use the faucet to make the symbol visible.

The house hot water system is not operating. You should fix it.

Go into the basement and fix the water boiler.

Get back to the bathroom and use the faucet. The symbol becomes visible.

That symbol is one of the symbols needed to open the cryptex. ( Open the wooden box in your inventory to find the cryptex. ) You need to look at the symbol.