Defence experts indicated that during the three-submarine deployment, HMS Vigilant was conducting sea trials following extended maintenance while another submarine prepared to take over routine patrol duties.

From the articleWhy are three of the UK’s four nuclear deterrent submarines at sea?Also in Why the deployment of 3 UK nuclear submarines is raising eyebrows, Three of UK's four nuclear submarines are out at sea in ‘unusual’ move: Report, Three of UK's four nuclear submarines out at sea