The sonic whale thing was an idea from a old-ish anime called Blue Sub 6.
I think the Dection thing is a better idea than full double blind. (Only tried once with the old harpoon rules, kinda fun but needs 3 peeps to work well.)
If you still want to add a bit of complexity stealth, then allow the units to move as blips untill detected (I.e rough contact. But you don't know if its a weak old sub, or modern HK untill you get closer.) You could also borrow an idea from some other games, and allow fake contacts to moved untill a sensor roll succeds and removes them.