I ran into something weird the other night - I was stalking a small freighter off the coast of Norway a few days after our invasion in April 1940, and was about to submerge to 'scope depth to get into firing position when my watch crew spotted an airplane. For some reason, I can't see the airplanes any more, so I just went ahead and dived like I usually do anyway.
A few minutes later, I popped up my scope for my first and only check just before firing, and I noticed that the whole stern half of the freighter was on fire and the mast that usually carries the flag was blown away. I thought I was seeing the aftermath of a Stuka attack, so I went ahead and finished off the freighter with my last torp.
When I went back to plot my course home though, I noticed the sunken ship indicator on my nav map was BLUE
Apparently, the plane was British and had attacked a German freighter - I'd never seen or heard of that happening before
I have never been in a position to radio in a convoy sighting or anything like that when I was anywhere near within range of any friendly air cover, but I do send in after action reports and have radioed in a few convoy sightings if I get a chance to do so before I submerge to attack, but so far it's always been way out in the middle of nowhere so I've never seen any response to the reports.