Stains on windows

If stains don’t come off with a razor or steel wool, what kind of stain may it be?

Does it smear more than come off?
If so, perhaps silicone.

silicone will come off easily with a razor if it is pushed into the surface texture of the glass. you can try steel wool, microfiber cloth, magic eraser. magic eraser is best. if that fails chemical for silicone removal.
other stains could be anything that is worked into the surface texture.