Yes, it might be a stain, but it might just be dust.
It’s one spot only, not the typical chain of stains, and the photos are too grainy to judge.
At the moment the camera is opened to put on a lens some dust may get in there. And a rocket blower does not "remove any possible dust". One might get lucky, but by blowing, more dust might get sucked in there or old dust shifted around and ending up on the censor.
I think one should at least take the time to do some proper shots (doesn’t take that much time).
Manually set ISO 100. Also, I think it’s better to manually set WB to indoor light (get a more grey/white shot instead of yellow). Just point at a well lit, preferably white surface (a sheet of paper on the floor, and move in so the sheet covers the frame). Don’t need a tripod or anything as it doesn’t matter if it takes 3 seconds and the camera is shaking - we’re not after a sharp photo of a wall.
Set focus to manual, and take one shot at infinity and one at closest focus. In the shots bellow it’s easy to see how the stains look different at infinite and closest focus. At infinite the large stains shows as black spots with a grey halo around, while the small stains shows up as blury dots. At close focus the large stains shows as large black dots with a dim light halo around them, while the small stains now shows up as small black stains with a grey halo.
F22 - infinite focus at top, closest focus at bottom: