[LeTharg]

Thanks for the feedback. I tried a little experiment and I don't think you can sweep an ally's mines. I'm not sure why not; maybe I wasn't doing it correctly. Anyway, I didn't even know there was a sweep button so I learned something.

As to how it happened, it has to do with planets changing hands. Mines in orbit don't go with a gifted planet. This particular planet has been through several hands.