I think, if I understand you right, you could just set up a trigger where if the frigate
detects the submarine, you could play a depth-charge sound and then you won't be damaged (but you're still 'caught')
I sometimes include a message with this in the trigger like "XO: Conn, XO. We've been counter-detected."
Also, if you Google the
International Code of Signals (1969) revised 2003 book, page 152 has the message procedure for the detection of US warships (submarines?) in Russian waters. That's fun to 'use' in radio messages upon counter-detection too!
It's also fun to 'escalate' the scenario if you're counter-detected - say, use a trigger to 50/50 whether the ROE is war/peace, set roe to war 30 minutes after detection, launch ASW aircraft to area after detection, etc.