If you're running speed is too fast (especially at periscope depth), even if you're outside of the circles =
You'll get a
"We've been spotted!" message -
Additionally, don't forget about the airplanes -
Ask me how I know...
EDIT ADDED / 04.11.2013:
If you send a weather report once you enter the "mission area" =
It will get intercepted and you will get "spotted" -
Initially, you can run at 8m / "semi-surfaced" / That way you can close faster & dive quicker when needed -
As I recall, I learned that from an old WWII sub movie / I just can't remember which one -
Run about 15-20m deep to get REAL close -
If there are no warships escorting the convoy, the convoy won't/shouldn't hear your screws turning...
Plotting & situational awareness is VERY important @ this stage -
I just completed a 2x recon mission -
I had all but 2 ships ID'd / The ones I need to ID for the mission -
Those 2 ships were on the opposite side from where I was =
I had to maneuver to that side to ID them =
I ran 15-20m deep, scope down (but pointing in the right direction) -
I'd come to full stop & "prairie dogged" -
Rinse & repeat as necessary -
I'm not sure if I was in my A or B boat...
I headed out of the area & set up for the Send Contact Report (as I surfaced to 8m) -
Luckily for me I got the Mission Completed (primary + secondary) just before I got "spotted" coming up to send the CR -
Back in the day...
I use to practice (and got quite good at) head on bow shots on DS's -
That one I got from the movie
=
Clark Gable & Burt Lancaster VS Bungo Pete in the Bungo Straits...
I did see an in game chat the other night that said this tactic doesn't work in SHO @ a distance of 500m -
I think 700m was my majik number + the semi-surfaced running -
Get target lock (or shoot from the hip... can't remember which) fire, down scope, crash dive, full flank & pull a hard turn... (as I recall...) -
You only get one shot at this... But it could save your boat -
I did a lot of elbow bending back then... 
So my memory is a bit foggy -
Good luck out there! 
I hope the developers add a "view lock" feature + pan buttons -
I sent BB Support those suggestions w/screenshots