2021 Discovery LXE 40M

Every few days we have to clean a dusty residue that accumulates on the rear bathroom counters (and probably all other surfaces).  I can only think of two possibilities. A/C unit or outside dirt from road or engine. A/C unit return filters are clean. I will be keeping a closer eye on how this residue is appearing but thought I'd ask to see if anyone else has experienced this issue. Thanks!!