There are going to be few U-Boats, in and around the Brest area, other than close inshore with an escort. These will be, boats proceeding out to their patrol areas, and boats returning from patrol, plus any carrying out tests, very close to shore, after maintenance. There is no reason for U-Boats to be in the area, there are not any targets for them, the targets are much further out to sea. Even the conveys, coming up from Gibraltar, will be much further out from the French shore, to be beyond escorted air attack range. Ships heading for the south coast ports, Falmouth and Southhampton, would split off out at sea, and pick up a costal escort, if that is ocean ships were docking in Southhampton at this time. Ships for the east cost ports and PoL, went up and over.
Even if by chance a U-Boat, observers the carrier group, British or American, and that isn't going to be easy to do. Best chance is to hear them while underwater, and proceeding on batteries, not while crashing through the waves on the surface on main engines. No way are they going to get close enough to actually see, what's what, not in the short daylight hours, and problematic at night, this is a high speed navel group, not a slow plodding merchant convoy. And sends a sighting message, what does it say, and what do the command ashore make of it. Note at least the British will know that a signal has been sent, good chance from where, and even if they can not decode the sighting signal, they should be able to decode the reply. It's going to take luck for the Germans to put everything together, and be prepared for an attack on the twins in Brest. There are lots of reasons why there should be a major navel group in the area, and it would take time, to work out which. Based on a single sighting report, that is singularly lacking in details. First thing command is going to want, is more information, information that is going to be hard to get.
Yes in retrospect, it will all come together, various reports from numerous sources, will with the benefit of hindsight, tell the tale. But that's for the Germans to do after the attack, as they try to find scapegoats for the disaster.
RR.