The coolant reservoir isn't strictly an overflow tank. You should check the level when the car is cooled down because coolant can still be in the lines and not in the reservoir or radiator right after you shut it off. Once the car is cooled down, the coolant level should be in between the min and max fill lines.

If you're checking the level at any time other than when the car has had a chance to sit the reading is not necessarily going to be correct. If it does appear that you're losing coolant level then you're going to have to go leak hunting. A small radiator leak (assuming it's not a hose problem) can be remedied by getting some radiator stop-leak goop which can be found at any auto parts store.