Last weekend my cabin heater stopped working and in troubleshooting that I saw people saying that low coolant level could be the cause. Both rear and front heaters were blowing cold or luke warm air. I filled my reservoir, which was a little low, and after a short drive the heat was working again. I haven't driven it a ton since then so I don't know if it is permanently fixed or not yet. Temperature gauges are always spot on 210 or whatever the center mark is once the engine warms up.

Here's what I need help with:
For the last couple of years I've noticed that the coolant level in my reservoir is about 1 inch below the cold line. Whenever I change the oil, I will usually top it off but it continues to drop to about that 1 inch line and never really any further. I can't find any visible leaks and my oil always looks clean. Why would the coolant keep dropping?

 

pressure test the cooling system and see if you get a pressure drop. If it does drop look for external leaks.

typically they leak from the water pump gasket.

 

It could also be the intake manifold gasket. When they leak they will drip down the back of the motor.