It's very interesting. I didn't come across any information about active sensors need fuel. Why actual mines are not dedecting though?
Active sensors don't need fuel. For the specific part of the mine, you need to flag no engine in missile design so that you would have constructed a buoy. On top of the buoy as second or third stage your will have you actual missile (mine)
If I read properly what is happening is this:
1st stage buoy (no engine no fuel only sensor)
2nd stage engine and fuel to reach the target plus active to get direction (here the tricky part, the missile will engage at a certain range only which you will set in the separation range box as the ships are passing through you don't want too close or too far)
3rd stage engine fuel active and warhead to obliterate (again with separation range to keep in mind)
Looks like the above works fine however, any different designed I tried doesn't work. Example:
1st stage long-range missile to waypoint for deployment
2nd stage buoy
3rd stage missile
Looks like the above is recognized as cheating design and intentionally not working. So you will be able to launch the missile, reach the waypoint (out of your MFC) and use the sensor. Once a target is in range the missile will fire but will remain static on the waypoint either it's still there or it has been removed from the tactical map. I have figured that as it recognizes the launch of a missile which will have to be then operated by an MFC the missile simply does not know where to go. To get this design to work you need the buoy to be launched as intended then you will have to use the buoy sensor to fire at target which has to be still under your MFC. Pointless pretty much, at that stage better have a ship or a fleet all equipped properly.
My guess is that the code recognized a "mine" only under the "launch ready ordnance" command but not if you fire the missile. I know it's weird but I spent a day to test this for another post where somebody wanted to use a very long-range missile.