Have someone pressure test the system with a coolant pressure tester. That WILL find any external leaks and can help find INTERNAL leaks too with a little more work if done by an experienced technician.

Whoever said that a pressure test was "no help" probably didn't have it done by an experienced technician who wouldn't give up in a few short minutes. Even with a pressure tester, if there's a slow leak, one needs to keep it under pressure for a while (possibly hours) and carefully look everywhere. If there's a pressure or fluid loss, it's going somewhere and the source can be found with a little work and determination if not blatantly obvious.

The idea of your water pump seaping so slowly that the coolant doesn't hit the ground just doesn't wash. Even IF you see a little coolant crust on the "freeze plug" just below the pumps pulley, do not replace the pump for that reason in itself as that's not a problem nor unusual.