All of a sudden, it makes sense. Edit
At first, I couldn't see as to why Masashi could be careless by allowing a sonar ping to be sent from his group. It wasn't until now that I truly realized Masashi intentions after rereading a couple of chapters. Right now, that random sonar ping doesn't seem so random anymore. Himiko had been on the run ever since Day 2 and Masashi had always been on the move from that very same day to recover his BIM and the supplies she and Mitsuo had taken from him. When I think about the location where Ryōta met Masashi, it makes even more sense of how they came to meet. I had forgotten that that was the location of where that they had come from, and that komodo dragon(?)'s corpse couldn't have made it even more obvious. There were so many traces of Ryōta's group that Masashi could've found there. It must've have been only when he either thought he was close or had lost them that a sonar ping was sent.
As for Sōichi, I can't say when exactly he met Masashi, but he might've met him somewhere in that very area. After all, that area wasn't too far from where Sōichi was last seen before reappearing at Masashi's side.