The reservoir shows under low. I fill up to full. After I drive half an hour, the liquid is back at just below low.

The engine is not getting overheated.

The problem is that I can't get any heat in the cabin. The fan is blowing cold air. Now we're talking about outside temperatures of zero centigrade and below. After driving at highway speed for 5-10 minutes, the heat starts coming. Then the engine has been running at least 10 minutes or more.

The heat should normally, as you know, come after just a few minutes.

I cannot see any traces of leakage. Not on the ground under the car, and not in the engine bay.

The car is RC300H with a 2,5 liter straight four, so it can not be the valley plate issue (only V-engine problem I believe).

 

You may have air in the system which causes your heat to fluctuate on and off (which can change when you accelerate, decelerate, uphill grade, downhill grade etc. Follow a bleeding process and try again. Can't find the leak? Did you take all the bottom covers off? On my RC, that's how I found the where puddles of pink coolant were accumulating- on the cover and absorbed by the sound dampeners!

 

I had a similar coolant leak recently and had to replace the water pump. I initially had a puddle under the car. Before diagnosing the water pump, I topped off the coolant and didn't see another puddle so assumed it leaked out while driving.