FWIW, for troubleshooting slow coolant leaks, I've found it extremely
helpful to use a cooling system pressure tester.  (Can get from
Autozone Loan-A-Tool, etc.).  Pressurize the system to 15psi or your
cap pressure, and wait for a few hours or more, pumping back up as
needed to maintain pressure, and look for leaks.  By pressurizing the
system while cold, the leaks will collect and not immediately
evaporate, giving you a better chance of finding them.

-Alex, who has found but not yet repaired the slow leaks.  :)