When you say drip marks is it a pattern? Like paint drips or is it run off from the surface above the window? Is it always from houses with the same exterior? Is it new or old glass? And it is always brown?
Doesn’t seem to have a pattern really, could be on long vertical crankouts or older double hungs. It always seems to drip from eaves though, and always hits the glass in a teardrop pattern as if it’s somewhat viscous. I really don’t think it’s caused by insects or rain drips.
Etched surfaces cannot be removed with a razor. I would look at the point of origin of each drip line. If some drips begin in the middle of the glass then some liquid probably struck the window. If they start at the top of the window (as you have said) then they probably came from some construction material. Or at least run off from above. I would look closer. But it is not always so easy to figure out. I usually carry a little bottle of my microcrystalline silica compound that I call SKRUB to test out different stains. It will usually eat through most. But if the stain is an etch it will not touch it. Most times. I also like to see if the drip is an organic or mineral deposit. A powerful organic solvent like EB, or an alkalie such as sodium hydroxide/oven cleaner will take off an organic but not touch mineral stains. Inspection tools.
Could be any number of things. Sometimes it’s leaching from stucco or stone siding. Sometimes it’s that from a/c drip or rust from gutter. Sometimes the silicone from the window can reach out into the glass and it looks like runs and stain. Start with steel wool. If that does not remove it then it’s will need chemical reaction to remove the stain or mechanical abrasion with a pad and polish by hand or machinical polisher.
Discovered a similar situation this morning to the OP; the homeowner pointed out some scratches on the glass from where the painters had sprayed lacker all over the glass and assured her they would get it off. They razored and scratched these big beautiful panes all over. Aside from the scratches, the glass cleaned easily. Then on the glass next to it, same style, same manufacturer, same surroundings, no scratch’s. HOWEVER, the glass had these smear looking etchings all over the place, cleaned horribly. Tried everything, white pads, 0000, all the standbys.
My thought is, they started razoring, noticed they were scratching the glass, switched to chemicals and left em on the glass too long. Not sure though what would eat glass like that. Wild stuff.