GRU was already aware that the Dutch sub, after Indian Ocean regional ops, had then transited the Suez Canal from south to north with a known time of emerging into the Med. This made it quite easy for a Kilo (from the Black Sea fleet) to cue or "rendezvous" with the Dutch sub. This Kilo gained valuable intelligence on characteristics of some upgrade work done on the Dutch sub.
The US Virginia class SSN had earlier been (predictably) tailing the Admiral Kuznetsov carrier group from a range of 70km. As the Virginia class sub transited the Strait of Gibraltar (traveling from west to east into the Med) it was detected by a fixed seabed sensor array (perhaps in cooperation with local assistance). The detection was later confirmed by Russian helicopter dipping sonars (active) and ship bow sonars.
GRU further reported that, by their behaviour, the Walrus and Virginia class subs positively detected the presence of the Kilo subs.
- Russia set to unleash carrier aircraft and SLCMs on IS in Syria of 31 October 2016, and
- Russian carrier Admiral Kuznetsov may conduct first airstrikes (against IS), October 17, 2016.