I just wondered if my mind is playing tricks on me or if the battery drains with no use.

Scenario: On Thursday I drove to work and home and ran a few errands in the Civic. I parked it Thursday afternoon to go on a trip with the family in the van. We left Thursday afternoon and arrived home on Monday. The Civic was idle all this time. When I went to run errands today I drove the Civic. The battery charge was down around half. I swear that it was full when we left on Thursday. Has anyone else experienced this?

Billy