This happens on my car, but it could happen in any home window. Therefore, I post here.
My car has some unknown sticky dirt on the window. I have no clue what it is. I can remove some of the dirt by my finger nail but not completely. I tried to use wet paper towel but does not remove any at all.
Note: I took two pictures. One is from the inside of the car. The other one is from outside of the car.