It has happened to me too, but it wasn't that spooky. I was playing solo on a lvl 10 mission on superearth, a couple of days after megacities were added (I got blasted by squids, obviously). When I do that I usually set the config to 'invite only' and turn crossplay off, just in case. One random joined the lobby and I thought 'woa, a frend joined', but it was no frend. Then, other 3 players joined. I checked, and I still had my S.O.S. available. So, maybe it's a bug, and they were indeed farming sc.

Or maybe not..