“Ghost” AIS targets (that patently I was about to crash in to)

[Roberto]

It might have been one of those fishing net AIS transmitters (usually Chinese origin), their transmitting signal can be set up to several minutes intervals; if they are very near the water level, say on a pot pole, you might have received its transmission while inside its limited range, then sailed outside their range before it sent the following signal.

Just checked, this sort of AIS beacon can be programmed to send its signal with intervals up to 10 or even 30min, depending upon models.