I have a feeling that this is a bug with TC. When I was new to SH III I had the same problem a couple of times. All cases had one thing in common: High TC.
Since I read somewhere that high TC is somewhat cheating because of the lower chance to be spotted by aircraft I started to play with a max of 128 at day and 256 at night (no planes around at 1940
). It had been foggy for weeks and I spend most of the time at 60m to listen around. I've been travelling the waters submerged at 60m for days at 256 (with breaks for resupplying with oxygen and recharging the batteries of course) without incident and it wasn't until I went up to 512 that something happened.
