Tom Quinn was declared a rouge officer by Harry and decomissioned early in season 3. Harry mentions later in season 3 that Tom and his CIA girfriend started a top secret, private mercenary-type agency. When we see the Home Sec., Towers, talking to Harry on the phone near the end of season 10 saying he understands that Harry hired an outside source to take of things and then see Tom appear in Russia at the KGB leader's door, it is implied that Harry called Tom to kill the Russian. In one of the extras on the DVD, Jane Feathersone (Spooks creator and Kudos executive) says that bringing back Tom (even for that 10-second scene) was done as a nod to the long time fans. I found his brief appearance a joy and appropriate for the circumstances. RIP Spooks! Even if it was not the ending I wanted, I will miss the series greatly!