Coolant level is much like brake fluid level and not like oil level.
Any drop in the brake fluid level or the coolant level is an indication of a leak.
If you are a DIYer, you can rent for free a tester at your local Autozone, to pressurize your system and find any obvious leaks.
The leak could be in all sorts of places from the hoses to the water pump to your head gasket.
Or you might be able to locate the leak under your car just by looking for the telltale dried white chalky stains of leaking coolant.