Sounds very possible, and that would explain the offset.
I examined my RZ67 Pro II back assuming that it’s similar to the RB67 back. When viewing the film opening with the dark slide removed, it’s easy to see that the pressure plate is about a 1.5mm deeper than the deepest edge of the short ends of the film aperture. That provides plenty of clearance for light projected at an angle to pass under the ends of the film aperture and strike the film beyond the usual frame boundary at the ends of the frame.
When image-forming light from the lens is projected back through the film aperture, the maximum angle that light can travel from the rear lens element passes through the aperture and defines the image rectangle on film. This is always about then same size due to the same (or nearly the same) projection angle of the various lenses that fit the camera.
But in the case of spill light reflecting from the interior of the mirror box past an out-of-position light baffle, the light can travel from the sides of the mirror box past the edge of the baffle obliquely to pass under the ends of the film aperture due to the 1.5mm clearance between the end of the film aperture and film lying against the pressure plate.
The source of the light is most likely from the lens to the mirror to the focusing screen. Some of the light that strikes the focusing screen must reflect and spread sufficiently to slightly illuminate the interior of the mirror box during viewing. Normally this is no problem if the light baffle is sealing the opening to the film holder as the designer intended. But if the baffle is not fully closed, then some light from the mirror box can bleed thorough the gap between the baffle and the chamber to expose the film—including a small area beyond the usual border due to the clearance mentioned. And, since the light still grazes the edge at the end of the film aperture, this would account for the sharp boundary, even into the area beyond the usual frame boundary.
Your explanation is without film in the back. With film in the back, that 1mm difference is taken up by the thickness of the film due to the pressure the plate exerts. Or, did I misunderstand you?
I just had a look at my RZ back and there's a good 4mm between film gate and pressure plate; plenty of room to have an offset between flare and image. I can also report that I get offset-flare (on two different RZ bodies) just like you've reported when shooting strongly-backlit scenes*, so it's not a fault with just your body. My guess is that there might be something small and reflective on the back of the mirror and that if that is illuminated during exposure then you get flare.
And it's clearly not a problem with the baffle - the sharpness of the flare image shows that it was formed only while the shutter AND baffle were open, i.e. during exposure. If it was light leakage during handling, the flare would be spread around due to camera motion.
* with the sun in-frame, you don't see the offset because the sun-image is on the film, so you just get lens-flare. This problem occurs with the sun only just out of frame, which causes it to illuminate the inside of the body. My scanner (Nikon 8000) doesn't really go outside the frame boundary because of how the holder works (and I tend to crop out the unevenly-flared edge-bits before saving!), so I can't easily show you an example. But this is IMHO normal behaviour for an RZ/RB.
interesting. thanks for your help everyone. I am still wondering - the foggy area is offset from the image which clearly means the unwanted light is coming in at a more extreme angle than the light from the lens. This makes sense vertically, as the fog does not extend to the top of the image, and extends beyond the bottom.
However, it is wider than the image area on both sides, which would mean that light is coming at an extreme angle from two different directions but creating a fogged area with a uniform (and sharp) top and bottom border. Can that be explained by flare?
I assume flare light to be multiple reflected and not fulfilling the requirements I put up in my post above.