[donut]

x2 on the cold leak. Warmed up parts expand, leak gone. Warmed up it may still be leaking, just evaporating before you see it.

Had it happen on a chrome water neck that didn't want to seal. Also on the water neck where the rad hose connects, corrosion can build up on the water neck preventing a tight seal on the hose. I seem to have this happen more on OEM necks.

Corrosion I usually clean off with a piece of plumbers cloth, if it's real bad, I'll use a file to get it close, then finish with sandpaper.