Occasionally I notice darkening in the corners of prints. This usually appears when there is a lightish tone (such as sky) in that area.
It doesn't happen all the time, and is usually just in the corners, in fact usually just one corner.
I had put it down to safelight fogging. But I just did a safelight test, with two coins on a piece of paper for five minutes, and there was no image at all, so that seems to be ruled out.
Any other possible causes?