[onelifecrisis]

I've done some reading on night surface attacks. Apparently, entire wolfpacks would perform co-ordinated attacks on the surface at night. Given that only one of them would need to be spotted in order to put the enemy on high-alert, I'd say that's a good indication that subs were very hard to spot indeed at night.

Hitman once read an account of a u-boat crew being able to hear the conversation among the crew on a target ship during a surface attack, they were that close.