Make a computer player that is friendly to all other players and has shared LOS enabled. Create a helicopter-type unit, one for each special unit, and have a looping trigger that tasks it to its assigned special unit on a loop. Then use a graphic effect to shrink the unit scale of the helicopters down to 1% so they cannot be seen. You will then have a permanent mobile reveal effect around each special unit. When the assigned unit dies its follower must also automatically be killed off to cancel the reveal. For extra safety, use an effect to defeat this additional player at the start of the game so that humans don't see it in the list of players on the Diplomacy menu and they can't try to go hostile with it. AI for that player should also have 'Active' switched off. For best results you want a unit that has a small turning radius so it is easy for it to keep up with the unit it needs to follow if that unit zig-zags a lot, maybe like the Hot Air Balloon.