I had this kind of problem with a chromega E, though it wasn't a circle but 4 dots of added density in the shape of a square that showed up in prints. Turns out there were four bright screwheads within the bellows assembly that were somehow reflecting down through the lens and onto the print -- very strange since they weren't in direct line of sight of the lens, but apparently light bounces around in very strange ways up there.
If your enlarger uses a lens cone, I'd be suspicious of the point where the vertical sides make the abrupt turn into the mounting ring. That's the kind of edge that can catch the light and form a bright ring that somehow reflects down onto the paper. But basically it's just a matter of taking off the lens (or looking down through the negative stage) and hunting for any source of a bright circle and flocking it.