Car losing coolant?

Hello, I noticed the other day my temperature gauge was elevated. It was not in the red, but it was headed in that direction it appeared. So i pulled over and checked the coolant reservoir levels. I couldn’t see any coolant in the reservoir so i added some to the reservoir. I filled it up when the engine was cold to the minimum line. I have noticed after a few drives and adding more coolant since then the coolant level is no longer at the line, but barely visible when i look at the reservoir. I've added maybe 3/4 of a gallon of coolant. My car hasn’t run hot, except for my previous experience, there are no visible leaks, and no smoke or coolant smells. Is this necessarily a leak? I am assuming my mechanic checks the coolant levels when i get my oil changed. Would evaporation cause this amount of coolant loss? Just trying to prevent any major problems. thanks