If I fill up the coolant to the max cold line level, within a few hundred miles the coolant level will be about 0.5" down from the line or about 250ml short. Then it stays like that without going lower. What could be causing the issue? Is this normal? The only evidence of leaks I see are some drop spots below the overflow hose. If I tap on the overflow hose when the car is at full temp, I can get a small drop of liquid.

The cooling system has been updated over the past 2 years with everything that that touches coolant replaced or restored. The system has pressure when the car is running and loses pressure when the car is off. Temperature is stable 183-194 (180F thermostat). Everything is running smoothly and there are no visible leaks around the usual suspects, TBH plate, head gaskets, etc. I see some condensation on top of the overflow tank when the car is running and the tank is probably the ONLY piece of the system that is still original to the car. Yes, the radiator cap is brand new ;-)

When I turn off the car, everything is normal, i.e., no waterfall noises, no odd things happening in the airflow tank, no "peeing." My hypothesis is that the car likes to have less coolant and the excess that I put in drips out for whatever reason.

Any ideas? Thoughts? Thank you,

 

Could be a leak elsewhere...
Could be trapped air in the cooling system finally being released...
Have you checked your engine oil for coolant?
Pressurize the cooling system to see if it will hold pressure.

 

If I had a leak, would the level not continue to drop off until I ran out of coolant?

 

I would remove your coolant reservoir and check it for cracks above the cold fill line. It doesn't have to be large, but it's entirely possible that there's a pinhole leak that coolant is being forced out of under pressure.

Normally, the coolant system is a closed system that should not lose coolant, and any loss is cause for further investigation. While there is an overflow hose, that should only discharge if the pressure in the system exceeds the capability of the radiator cap, which is also a problem if that's what's occurring, especially with a new (and presumably to spec) radiator cap.

 

I think you're right. There may be a crack at least one place looks suspicious. It is also makes other things possible--air gets into the system and condensates on top of the overflow tank, then comes out. Pressure-wise, the usual suspects--e.g., waterfall--have been ruled out.

 

You can get a cooling system pressure test kit on loan from most auto parts stores. You pay for the kit (it was $250 at AutoZone this summer) and you get the full amount refunded when you return it. It's very easy to use and in the case of my truck there was a leak in a weird spot on the reservoir that the test kit made very easy to find.