It's not one big light source but a very small light source that leaks out through a tiny gap in that rectangular "thing" that seems to be related to the shutter mechanism. As I said previously, it doesn't affect my own modified Z6.
Just to be clear, you did try "Silent Photography"?
There's another experiment that would be useful. If you do a 5sec exposure, is the anomaly as bright as a 4min exposure? That will tell us if the IR source is continuous during the exposure or if it comes on momentarily.
There still remains the slight possibility that the lens itself has an IR module built in. To eliminate that possibility, try another experiment with tape over the electrical contacts to prevent power getting to the lens.
Correct, I tried silent photography on and off and here are the results. Just like before I wrapped the camera in a heavy blanket, placed it in a black duffle bag and inside a dark closet in a dark room at night.
Here is the camera with the 35 1.8 mounted, silent shooting off. 240 seconds, f/4, ISO 800, exposure pushed in post.
Same settings, silent shooting on.
Here is the same test but with the 50 1.8S, a lens I am having no issues with. Same settings.
Next, here is the test with a shorter exposure to see if the IR effect is any different. 35 mounted, 5 seconds, f/4, ISO 800, exposure boosted in post.
Same test, but with the 50 mounted. Same settings.
Now, some tests with just the body cap and some aluminum foil.
480 seconds, ISO 800, exposure boosted in post, silent shooting OFF.
Same settings, but with silent shooting ON.
I will try taping up the electrical contacts and the "point of entry" for the IR light source next.