I tend to agree, the "disappearing" trawler was odd, no coastal cover, no other bigger ships around, the returns on the scope seemed devoid of any significant clutter?
"You canna change the laws of physics, Captain". Looking at the range of the Helicopter from the ship, the AGI would have been beyond the radar horizon of the Duncan and was either being tracked by an airborne platform that had gone off station, or the AGI had switched off its AIS transmitter (I assume that they will normally transmit unless operating covertly). At the end of the day, there was a NATO taskforce out there, and the AGI had slipped out of force coverage. No big deal and nothing unusual.