You can lose some coolant through evaporation out of the expansion tank. But you may be dealing with a small internal or external leak, that might not reveal itself unless you drive a few miles at a time. This may be the same if you accumulate enough short trips. Sometimes it's useful to add some dye to the system to make the (external) leak reveal itself. The radiator cap is another potential leak source. System pressure (for very small external leaks) and combustion (for internal leaks) tests are useful as well when trying to locate the leak source.