Something similiar happened to me last week. I checked on about a half dozen hives. Drove up to the next few hives, and when i came back to my truck there was a big fat queen chilling out on the back of my seat. I never did find out which hive she came from. Queens are clumsy, ive spotted many of them crawling on the ground after inspections. Perhaps you had a swarm come and then leave and one of the virgins was left behind.