Confirm the location
Record the room, building layer and what sits above, behind or beside it.
≋Dark marks
Visual Match pattern
Gray smudgy window film
A smooth wipeable film may be soot or airborne deposition.
Visual resemblance is only the first clue.Location, timing, texture and what makes it change are more useful than color alone.
Decode this pattern →Watch the trigger
Rain, water use, temperature and equipment cycles separate causes.
Verify before repair
Use the next distinguishing check instead of treating the surface appearance.
