Best thing in the world for finding coolant leaks is a pressure tester. You can pump it up to 16 psi and feel around for the leaks. When the car is hot the leaks just evaporate.

I fought a leaky thermostat housing for years and finally got rid of the gasket and just used water pump silicone sealer. I found when using a gasket that the aluminum housing just bends as you torque it and becomes curved and then leaks.