"Prepare to be surprised" -- I should put that on a sign over my bunk! Case in point: Last night I did my second attack on a convoy 300 mi. west of the Continental Shelf. The convoy was originally heading east (prob. to Liverpool), but turned south after my first attack. It took forever to find them again in the dark, partly because at first I refused to believe they made such a sharp turn.
Eventually we caught up with them. Suddenly (it's
always suddenly), a DD came out of the darkness, heading
North -- & passed by without seeing us (WHEW!). We pressed on and made a good attack on the merchants despite stormy seas & lousy visibility -- sending slow torpedoes to the ships farther away, to give us enough time to flee.
We escaped east at flank speed; we were all watching the fires far behind us, and a distant star shell, thinking,
Yes, we made it! Then I glanced to starboard and
EEEK!!! Another DD, coming out of the east, heading west towards the convoy! Or was it the same DD that passed us earlier, coming back in a loop? It was like 800 meters away from us!
Heilige Scheisse! 
On instinct I ordered a crash dive, but cancelled that when I realized the DD didn't see us -- no searchlights, no star shells. I speculate they must've thought we'd attacked from the other side of the convoy. But that taught me -- don't assume you're safe -- if you haven't accounted for all the escorts, there might still be one out there, heading your way!
Mach's gut!