Where do these contacts come from?

[bossfrog]

I'm confused. I have realism set to 93% (everything except the external turned off) and I'll be traveling on course when all of a sudden, a red or green contact gets drawn on my navmap. They don't update or anything. They just sit there and gradually fade. They're drawn at a range of up to 180 km away with no explanation. My sonar guy isn't detecting them. They're WAY beyond visual range. I don't have radar. What gives? Am I supposed to believe some guy in a canoe paddles his way up to my boat unseen and tips me off to the location of some juicy prospects? Even if I got a radio call or something, that would make sense of it, but nothing. They just spontaneously get drawn on there. Can someone explain?

Thanks.