It doesn’t appear when there’s no negative in the carrier - my thoughts are the light is spread enough it doesn’t show up?
Hm. That's an interesting clue. Just to be sure - you don't get this on a test sheet processed without the sheet having even been near the enlarger? Because the first thought is that it might be something like fogging early on in the paper path of the RCP20 due to room lights being turned on as the paper is making its way into the developer bath. The RCP20's lid isn't perfectly light-tight in my experience.
In any case, I just wanted to verify that the problem is without a doubt attributable to the enlarger and that all other causes elsewhere in the process have been excluded.
If the problem only occurs if the negative is in the enlarger, I'd expect that there's some kind of light leak in the enlarger itself that is relatively well-focused on the negative stage. Perhaps some light from the bulb leaking past the light-mixing chamber or past a diffusor? I'm not familiar with your enlarger, so I couldn't give more specific clues; sorry.
Those four overlapping tan prints; are those made with a blank piece of C41 in the negative carrier? If you change the filtration, does the color of the mark change along with the filtration, or does it remain the same?
PS: I wouldn't call this a 'grey' mark. It looks brown to me, and reddish-brown is consistent with white light fogging on RA4 paper.