I and my watchkeeping partner were on the Morning Watch on an overcast, and otherwise pleasant morning, but with only moderate visibility, and with not much in the way of traffic movements and I, as is frequently my choice, was standing on the West side of the Watch Tower thus enabling me to keep an all round lookout whilst also monitoring the AIS traffic movements as shown on the Ship Plotter. On one of my routine assessments of what AIS traffic was showing up on the screen I spotted the AIS data of a yacht on the Swashway apparently entering Portsmouth Harbour. Nothing unusual in that except that I could not make out any yacht in that given position even with the aid of my high power binoculars However, on my next and subsequent checks of this AIS target of a yacht on the Ship Plotter I observed that the AIS target had apparently altered course to the North. This gave me no cause for concern as I just assumed that the Skipper had decided to change...