I had a mysterious leak of coolant. It was winter and I was losing coolant. I could fill the coolant up drive, forever w/heat on and check the coolant and it was fine. I would check the coolant before starting the truck and it would be half empty.
I have the habit of turning everything off in the vehicle before turning off the car, especially in the winter when I know it is hard to start in the AM. I turn off the heat.
The heat control valve for my rear heat was open while driving, heat on, and the valve did not leak but when I shut down the valve would close and leak in that position. I park on grass so I did not see any puddle of coolant. When in a parking lot the valve was still in the open so, no leak. I notice coolant on the frame and that gave it away.
Replaced the valve and no more leak.