I was thinking about having mobile mines. You build them on a planet, then can move them where you need them.
__________________
Creator of the Star Trek Mod - AST Mod - 78 Ship Sets - Conquest Mod - Atrocities Star Wars Mod - Galaxy Reborn Mod - and Subterfuge Mod.
Perhaps he placed mines where he had cleared your minefield, but the game got confused about the fact that they aren't your mines anymore. Have you tried moving over that sector?