Thanks for the input!
I deliberately went over 100 RPM btw to see if they would react; same with shooting my flak.
I don't know how to crank up the difficulty beyond elite..
I changed the date to 1944 once and they noticed me (Radar) at almost the same time my watch crew saw them.
In my current career (April, 1940) I'm about to intercept a convoy. In 13 hours it will be dark and I will probably find out the hard way that it's impossible to close a DD to 100 meters..
Edit:
Screenshot from the 'moon' mission: