I've seen a few posts on here about not being able to track down a small coolant leak. If you have any doubt on where coolant is leaking from, throw some dye in the system and poke around with the UV light. This stuff is cheap and it takes all the guesswork and hours of pulling your hair out of finding a pinhole leak. Even with the smallest leaks, it's almost literally like a neon arrow pointing right at it. Hope this helps.