AIS phantom ship

Yesterday we were sailing near Angel Island, when I looked at the plotter screen, and saw an AIS target moving fast, close, and on collision course (plotter shows a dotted line of projected course for AIS targets, the length of which relates to speed).

I looked in the direction of the target, and there was nothing there. No boats at all, not for miles. The target on the screen got closer and closer, until it passed straight through us - then vanished.

If this had been a foggy day, this would have quite freaked me out. Thankfully it was a nice sunny day.

Has this glitch been heard of before, or have we witnessed the first AIS ghost ship?

I wish I had had the presence of mind to select the target and see what the name of the vessel was.