The AIS is boat specific but it might not show the name (just googling the number will show who it is or clicking through to the details of that boat on some trackers) there are sometimes when they are switched off and off the top of my head I can't remember when that is. When I was posted to...
It could have been anti-fishing or coast guard which paid for by respective agencies but are run by the navy and in navy colours.
See a fisheries vessel regularly off Brighton as the patrol the coast. Off you really want to know next time search AIS tracker and see what's off coast.