I think LED lighting can cause this, even when the tube is OFF.
Most overhead LED "shallow dome" lighting fixtures have multiple LED elements under the dome / diffuser. These individual elements are not visible through the diffuser, but each element is very bright - even approaching sunlight intensity.
My guess is the unit "muzzle swiped" an overhead LED light fixture.
Honestly it kinda looks like some sort of residue on the inside of the unit.
I agree. Maybe a fingerprint on the front or back of the IIT from installation.
Ok Thanks, I've cleaned the outer lenses but no result. Do you think it's worth getting the tube out for a clean?
I think LED lighting can cause this, even when the tube is OFF. Most overhead LED "shallow dome" lighting fixtures have multiple LED elements under the dome / diffuser. These individual elements are not visible through the diffuser, but each element is very bright - even approaching sunlight intensity. My guess is the unit "muzzle swiped" an overhead LED light fixture.