I have heard about that before. THey do it all the time, especailly with tankers. Problem is that if you get too close, you can get sucked into the prop wash!! No doubt, that does not do much for your promotion chances!Both heading in the same direction, into port, sub fully submerged, no details. Hmmm....Sounds like the sub was trying to hold position under the ship in order to either sneak in under it, or shadow it without being detected.
That would be a conventional tactic--and danger situation. No news here.