I was so very very close...

[WernherVonTrapp]

I remember experimenting alot w/external cam when I was playing the stock version. I was trying to learn how the game behaves under various circumstances. One conclusion I came to was that, it's not a good idea for me to TC while any DDs are approaching. I could never resolve why the game behaved the way it did but I noticed that, at TC, for some reason, the incidence of detection by the DDs was increased, even though I had taken all precautions (silent running, no damage control team working, etc.). These experiments often involved saving and replaying the same scenario several times under varying conditions.
As a result, I never TC now when a TF or convoy is first detected. I remember in one scenario, I think it was during the convoy practice mission, I sank quite a few of the merchants, slithered away by the skin of my teeth and once I was (should've been) far enough to stop worrying about the DDs, (somewhere in the vicinity of 9000yds away) I TCed and immediately had the DDs zero-in on my exact location and promptly destroy my boat. Never did figure out how they accomplished that after I managed to gain all that distance undetected.
I don't know if these dynamics carry over into any of the mods but as a rule of thumb (or was it wrist -Boondock Saints), I just don't TC anymore while in the proximity of DDs.
BTW, you external cammed about 12nm? That had to take quite some time, no? I've done the same thing on occasion (though not quite 12nm) but gave up on it due to the time it takes at distance.