I am in a weird situation. When I drive my car inside the city then the coolant in the reservoir is decreasing but when I am on highway then it remains same. For example yesterday I drove around 150 miles on highway buy it remains same. As I mentioned it is decreasing from the reservoir. There is no visible leak and there is no oil in coolant and vice verse (I recently got oil changes that is how I know it).

A side note; a few days back I got the following error on OBD2 scanner "coolant temperature below thermostat regulating temperature." My mechanic changed the thermostat (it was stuck open) but the coolant is still decreasing.