I had a problem like this and it ended up being a hairline crack in the radiator end tank. Couldn't see the crack because it was too small, and coolant would only leak if it was under pressure or if I left it sit for a long time. Overflow cap, HG and hoses were all good. Replaced the rad and viola!

 

i have to inspect the radiator and we are studying on how to change the HG

 

well i got lucky they messed but my engine and they are replacing the cylinder head so my leak will be fixed at the same time if it was a HG