At 100 miles, you'll probably have to send an anti-submarine rocket and/or ASW helo out after it. In fair seas I imagine the wake created by such supercavitating object will be very prominent and guide the pilot straight to it's source.
Yes, that would certainly seem to be a pretty obvious arrow. Though at 100 miles out, you do have some time to run (certainly more time to run when launching at 10 miles and unless I understand poorly, that usually gives you away as well).
From the article it says you have to start with a large velocity, so the option of a quiet launch and then run before it gets up to ludicrous speed isn’t there.