I was nearly finished modeling a part when I noticed a plane that showed up at some point that I couldn't hide. Heck, I couldn't even select it to hide it. It does not highlight when I mouse over it. I saved a copy of the file and deleted every feature in the copy. The plane was still there. Tried on a few different computers here (SW 2017 SP5), the plane was still there. It looks like it is coincident with the Top Plane... toggling Top Plane visibility doesn't hide it... I don't know what else it could be. Anybody ever see anything like this? Any ideas?