Hm. Or maybe mines have to be activated or something by a ship/base in that sector? That'd make building other defenses of some sort on a warp point needed to work.
To-hit probabilities are a good idea.
Maybe make a sensor that can detect mines in the same sector and let point defense or normal mount weapons attempt to shoot down the mines. -THEN- go resolve the minesweeping attempt. I mean, mines can be placed in tactical combat. Why not try to shoot them if you can?
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"What do -you- want?" "I'd like to live -just- long enough
to be there when they cut off your head and stick it on a
pike as a warning to the next ten generations that some favors
come with too high a price. I would look up into your lifeless
eyes and wave like this..." *waggle* "...can you and your
associates arrange that for me, Mr. Morden?"