Periscope glass reflection from the sun? It wouldn't surprise me in this game and I take care never to leave my 'scope up for more than a few seconds at a time. Follow the link on the RN 'Perisher' course and you will see what I mean about proper periscope discipline. There was a BBC documentary about it many years ago. I remember it well and its lessons have been a great help to me in my SH3 'careers'.
In GWX there are some elite DD's with crack crews that have every sensor available running at maximum efficiency, unlike us.
They can DF radio chatter, pick you up beyond normal visual or sonar detection range and probably even 'smell' you underwater.
The No. 1 sub-killer, Johnnie Walker, had a team of ASDIC (and later Radar) specialists who had an uncanny nose for finding a contact and these guys just get better and more numerous as the war drags on.
Perisher link:
http://en.wikipedia.org/wiki/Submarine_Command_Course
Walker link:
http://en.wikipedia.org/wiki/Frederic_John_Walker
BBC 'Perisher' Documentary Part 1 (you won't be disappointed):
Take care, it's dangerous out there.
KH
edit: If we can 'cheat' by tweaking our boats and/or crews beyond historical limits, we cannot complain if our enemies do exactly the same...
