This is very common in the south, and many locals are like this. Some people even put it on the window of the security net, with the mirror facing outwards. It is said to be used to ward off evil spirits. Some geomantic omen say it is to reverse bad things, but usually the residents across the street are not happy.
No matter how you pay attention to Feng Shui, you should also respect others. If I understand it wrong, count me in.