The only thing I could think of, looking at that pic is that maybe it's not sealed right and coolant is slowly escaping from that top bolt as steam, which it could then condensate on the too portion. In your close up of the bolt, I see liquid on the top of the bolt. I would have it checked just to be sure, they should be able to pressure test it to make sure. But of course that means you'll be without your rig again for at least a day.