The initial sensor on the mine itself is a thermal one, the submunition has an active sensor. I believe the way it works is that the mine sends the missile at the contact it detects, and it is up to onboard sensors on the missile to find how much the target has moved since it was launched by said mine.
There is as yet no way to put in, say, a ship counter on your mines, so no way to have them be dormant yet. As for missile "fighters" themselves, they have a lot going for them, but are much more expensive in general.