GTI fan turning itself on after car has been off for hours

GTI 2008. A couple Sundays ago we drove to my in-laws and back (about a hour’s drive one way) with no issue. The next morning my husband walked the dog and when they got home he noticed the fan was running. The car had been off for at least twelve hours prior. Took to mechanic who said no issues with faults and they couldn’t replicate it. We disconnect the battery whenever we park now because we’re paranoid the battery’s gonna drain. No one seems to know what could be causing it. Has anyone seen anything like this?