Since i saw in the document how wolfpack used moonlight in night attacks
i tried to be less paranoid while attacking but i found strange things.
I set my radio to be able to listen radio communication 20k meters around (which in effect listens to whole area around england, not just 20k around me) i started to listen to their radio message periscope spotted at $my_location
1) someone (probably the ship which was closing) sent message they saw my periscope just 10s after i got it out of the watter on 3km+ length in the night (clear weather without wind nor fog) I was in the front of them
2) they did nothing, just flew along as they wouldn't see me
(if they didn't know about me, who sent "the periscope sighted on $my_location" message?) I was only sub around for sure.
In the document above, it was said, u-boats often hunted completely emerged in the night, so i would expect my periscope will stay hidden for few minutes at least in the night.