I had a slow leak from the rad at one of the transmission cooler nozzles. Only found it when the nozzle retaining nut split and leak got a lot worse.
On another car, I had a small leak in rad. Hardly any drip, but likely leaked while driving.
May be some like this. Just leaks while driving. But you would think this would show up on pressure test.

ADDED: Forgot to mention the water pump. Over the years I had 2 small leaks. One from weep hole in pump (on 107) and once on 300D from water pump flange gasket. Both very small. Probably vaporized while driving, but dripped over night. Discovered by placing large piece of plastic coated cardboard under car and checking it in mornings.