I had a similar experience. Whacked two troop ships and the escorts were good - DC'd me for an hour. It was very early in career (no promotion points yet for crew) and I finally got away, or so I thought, by setting depth to 112 meters (which meant that the sub actually held at 174) running minimum ahead silent. By experimentation, I had found before that going to 180 meters started causing hull damage. Again, this with totally nube crew. It really was intense!
Just like you, no noise on the phone. I went to periscope after two hours down there "in the clear," did not do a sufficient sweep with the scope and surfaced. Three escorts were up there, spread out in a big triangle, each about 7000 meters away (I think). I was soon spotted by two of them and the same horrific undersea drama started all over again, this time with 8% hull damage.
If only the AI were like this going forward. I am now around May 1941 and in all that time I have never again seen anything resembling competent escorts. Either the AI needs vast improvement or I have been very lucky.
