Here's something that happened to me a few patrols ago. It's late 1944 and I came across a lone, slow-moving C2 Cargo ship north of Ireland. I was at 25 m depth and decided to speed ahead of him for a torpedo attack.
Instantly my little detection symbol in form of a U-Boot (I'm playing SH3 1.4b) goes from green to red - the
merchant had detected me apparently. I was like "Huh?"
Just to be sure, I checked with my hydrophone man and consulted my map (F5). No warships in the vicinity. So how did this merchant detect me when I went to full underwater speed!? A game bug?