I was on a trip last summer and when we stopped for gas my passenger noticed an antifreeze puddle under the passenger side by the heater core.
I looked all over and couldn't find the leak. The coolant temp was 165 deg. I turned the heat on and off, nothing. Started pulling on hoses and found it only leaked when the hose was moved so either from vibration or pulling on it. It was the heater hose without the valve in it. Drove the car 8 miles to an Autozone and it didn't leak a drop on the way but touch the hose it was a different story and the split wasn't particularly small either. I never knew a hose could have an intermittent leak before.
Working on the car last week I was replacing all my vacuum lines and check valves( 2 of which were intermittent) and checking all my hoses and putting in a 180 deg. tstat when I found the S hose had a split in it and it wasn't leaking either until I moved it.
I'm doing a thorough check of ALL my hoses at least twice a year now.
I hope you find it's something simple.