Water pump leaks can be tricky. Hard to see and may fade in, and out. You may also have a leak thats aggrivated by driving motion, so it did not show up when the car was on the lift. I have also had minor hairline cracks in radiators that were hard to spot.
I would avoid the stop-leak / bars-leak. More trouble than they are worth, IMHO. There is a product that GM used to ad to Saturn Alum block engines. Don't 100% remember the name, (goldenrod??). I am pretty sure they still sell it at NAPA. Its small round gold tablets. Anyway, GM had problems with Porocity in some of their aluminum blocks, and this was how they dealt with it, so it can't be all that bad.