He doesn't know they weren't. He just knew how long the car was there. However, when the car was searched, there was only two receipts in it. There was nothing else in the car. One person in Jon's circle said that he had a ton of camping equipment that he took with him. This would be camping equipment suitable for summers in upstate NY. They may have met up with people in Big Bear. I think it's clear they planned on going there and perhaps he had been there before. Maybe he knew someone there. I don't know. At this point, if they were in Big Bear, they would have been verified by now. It's a small place. Even if she dyed her hair a different color. Also, how would they be surviving with no jobs or money? If someone picked them up, I suppose they could be living off of them. Who knows? None of it makes sense. Even if M. is running all over the country paranoid that people are out to get her - what is Jon doing? Why wouldn't he contact his family? Which is why it makes sense he is in contact with his family. If he is, why aren't they saying where he is? What is the big secret? It makes no sense. Did something happen in the woods? Are they even still together? There is nothing but questions and no verified sighting other than Burbank on 8/30.