Never saw the message 'unknown vessel'. Perhaps it's part of the programing of the unit when info hasn't been filled in but I always got many targets where the info window it only show position, speed, distance, etc. No info on the vessel itself.

Makes sense in a way, you know the vessels position, direction, speed and distance but nothing that will show personal information. We never had a transponder, only a receiver so the entering of vessel info is something I know nothing about. I will say that when we were cruising we never wanted any personal info transmitted. Could be the same thing or just lazy instillation. I don't think that personal info on an AIS manditory vessel is required by law, only AIS basic info.