re: Viking
Yes, the only flat area on the Viking Lander was the camera testing grids and if I remember correctly they dumped soil there several times on each Lander. So if dust blew away it would be pretty hard to tell.
Still someone with lots of time could look at the various images and might be able to tell if dust accumulated or blew away over time.
That area was also sort of surrounded by the RTGs, cameras, high gain dish, leg supports and, etc...