Coolant overflow

Hey guys, I got a question about keeping the coolant reservoir topped off.

Bought a project car and had the motor rebuilt. The shop that did the work is probably the most respected one in my area, specializing in engine rebuilds. Only have about 170 miles since getting the car back. Everything's been good so far, kept the RPMs low (never over 5000) and check the oil and coolant every morning if I'm driving it. Only potential issue I've noticed is that some coolant in the reservoir seems to keep disappearing. I've filled to the full line (cold) drove it a bit, parked the car, and the next day it's back to the low line again when I check it (cold again). Radiator itself seems full and I get heat. According to the Torque app, the highest coolant temp I've seen is 89C after driving a bit which is within spec for the car from what I've read. Car gets parked inside and I haven't noticed any puddles or weird drippings. Am I just overreacting? The Taco's coolant stays right above the low line and I've never had to add any.