A diesel submarine running on battery power is the quietest sub going. Passive sonar has problems detecting it even at a very close range.
However, a diesel submarine running quietly can in no way make enough speed to keep up with a Carrier group running under normal operating conditions.
This is probably a case of the carrier group going right over the top of the diesel sub rather than the sub shadowing the group. But it would be more embarassing and far less provocative to say that our equipment didn't pick up the sub.
Just my
