You said coolant reservoir, so you certainly have no lack of coolant in your car which is a good thing. For peace of your mind, add more coolant to the reservoir and watch the level every now and then. I don’t think you have any issues. Good luck. 

Was it really cold outside when you checked? It's normal for the coolant level to drop a little when it is cold (antifreeze contracts when cold and expands when hot). From what you described, you don't seem to be having any coolant burning or mixing with the oil. Now if it's a lot of coolant that's missing, then that's a sign of a leak somewhere. I would let the car idle up to operating temperature and check for any coolant leaks around the radiator, it's hoses, and the water pump.

First of all, keep the coolant topped up in the reservoir, and check the level of coolant in the radiator itself (only when motor has cooled down). Top that up, if necessary.

Then, if you still have to add coolant on a regular basis, look for a small leak.

You may have a small 'pin-hole' leak somewhere that doesn't result in an obvious leak that reaches the ground (like in the upper part of the radiator, or the reservoir itself, or one of the hoses).

I had a small leak in my old Integra, that was hard to find. It turned out to be a 'pin hole' leak in the top of the radiator, from which STEAM would come out when the motor was hot and the system was pressurized.