I just looked at the contact sheet again, (would love to post it - but I can't scan for the time being) and the light definitely changed. In my case I took four frames in relatively quick succession with the camera tripod-mounted and with the same lens. The difference in the frames was two with and two without a G2 filter and an exposure variation of 2/3 stop for each of the two "sets". The mark shows on one filtered and one unfiltered frame but it's clear that the light changed. (It was in the U.K. !) The more I look at it, the only thing I can conceive is a freak reflection into the lens from something. I was using the 100mm macro so I'll get that out later and see if I can see any bare metal on any part at the front. I also tend to stand right up next to the camera when it's tripod-mounted, so it could have been something I was wearing!
Many years ago I had a similar incident with a different outfit. It stopped me trusting the camera for a long time, though I did use it - and it never happened again. I'm not going that route again though. I'll just keep using the system.........until it comes back to bite me in the arse!