Do you have another [preferably smooth, such as drywall] surface you can temporarily aim the projector at just to quickly rule out anything related to positioning? Also, do you know the make and model of the projector?
Yep. I agree.I know nothing about projectors...but it weirdly looks like a reflection of some sort.
I'm not sure where you're going with this. Of course it is. If you can't re position the projector as previously suggested, maybe try holding a sheet of white paper in front of it as close as needed to produce a full screen image on the paper. There are only three components here, the projector, the screen and whatever is [or isn't] behind the screen. If you still see the unwanted image on the sheet of paper then the problem is inside the projector. If you don't then it has to be one of the other two components.It is still there with the lights off and we have been doing the exact same stuff for several years and it just appeared this Sunday- so not sure where it could be from.