I had a similar issue where I had 'mystery' coolant loss. I couldn't really tell where it was going. I could see a bit of spray under the car on the rear passenger side, so I fashioned an overflow bottle for my overflow bottle (ziptied water bottle off overflow overflow). I then took it for a drive. I figured if it wasn't blowing out the overflow that would be a good clue. What I found was that it was indeed blowing out of the overflow and when I stopped to look at it I could see air bubbling up in the bottle. That was a sad sight as I had confirmed my fear......blown HGs!!!
Confirmed with compression tests and visual on the spark plugs. And that was that. Engine out full rebuild (which I am still finishing up).
I hope you find something less serious because a rebuild on this engine was big job.