And the world ended that day...sarc
Crazy Ivan
It says the frigate initially picked up the sub on the towed sonar. There are two main benefits for towed sonar. One is to get the sensitive hydrophones away from the inherent noise of the ship. The other is to tow the array below the surface layer - somewhat deep - because noise (sub noise) from below the layer can reflect off the surface layer - thus being inaudible to surface sensors.
Presumably (?) the sub was running relatively deep. Not much reason for modern nukes to surface. A detection off the towed array would lead you to believe the sub was down at least several hundred feet.
But then there was a periscope sighting and a radar contact. Why would the sub come up that shallow? Even if it already was relatively shallow, why show your periscope - a visual and radar target? Maybe the sub wanted visual confirmation of what their sonar was telling them? Maybe they don't have a good sound signature, didn't recognize the frigate, and wanted to associate the signature with the ship.
If the frigate had the sub on sonar and radar, there would be no real reason to close on it. Particularly if you have your towed sonar out. British Captains don't tend to make those kinds of mistakes. I'd bet the sub initiated the close pass. Maybe it didn't count on a towed array. Kind of like maybe the sub wanted to get close and collect a real good signature. Odd because the Type 23 frigates have been around for 30+ years, and this particular frigate for nearly that many. They should know what they sound like just about as well as the Royal Navy themselves.
Why would the sub be so interested in a fairly routine frigate that they'd go up to take a look and then make a close pass? The usual nuke response to a potential surface detection is to go deep, slow, and quiet and simply disappear. Just seems odd all around.