I have used enemy ships as cover many times but I'm pretty sure the escorts always knew I was there. They just keep their distance to avoid hitting the merchant. This works really well with a Type XXI when you can slalom around and under ships of the convoy as you pick and choose targets. At 15+ knots the escorts can't keep up with you in the pack.
In fact, I have parked in shallow water next to a sunk-and-bottomed ship before and let the destroyer circle me - they won't drive in for fear of hitting the wreck. I then shoot at them as they cross my bow or stern tubes. Sometimes it even worked.
Steve