Hopefully it's not one of the more labor intensive problems mentioned above!

Also, something that would have helped me find it much sooner was looking with a mirror. All the signs of a leak were on the bottom side of the hose fitting, of course. I have a bunch of mirrors, but assumed a coolant leak would be easy enough to find without one, or at least with a pressure test. Sometimes the simplest of steps can make all the difference.

The way I verified my valley pan leak was with a little bit of UV dye in the reservoir. the seal around the valley pan lit up like a Christmas tree on steroids. This was especially helpful as I couldn't see the coolant pooling at the rear (visually obstructed, dark, etc). It also allowed me to check every single point, including the underbelly, for leaks.

I still have two gallons of unused (thankfully!) BMW coolant as I knew I was going to find more leaks, but haven't yet.