Coolant leak, but no idea from where...?!?

There's nothing embarassing about that! What I would do to start is to pressure test the cooling system...may not work cuz you say the leak happens after a hour of sit time, but its worth a shot. The second easy thing you can do is run to your parts store and and get your hands on some coolant tracer dye and pop it in your radiator, it will make locating the orgin of the leak much easier!