Do you think he knows her?

It could very easily be nothing at all normally when a girl stares at a guy it's instinctive nature to look back it's one of those protection mechanisms that men have built into them. Men are always on the lookout for a predatory animal than anyone stares at them their wife their kids they will stare back and watch that making sure that no harm comes to anyone. It could be that he does know he may have seen her at a ballpark or somewhere else around the neighborhood it doesn't mean anything bad necessarily. It's like that common meme where the husband and wife are laying in bed and she's like I bet he's thinking about this or this or this when really he's thinking about do I really want to paint the walls in this bedroom or something stupid.

Could have just been that sense of I think I've seen you somewhere...

They were probably checking each other out. When you are in the car all you see is a head and shoulders so they were probably just checking out each other's bodies. Its human nature. Just because you look doesn't mean you will act.

You are overthinking. People who don't know each other also look at each other multiple times.

More than just knows her imo met there intentionally

Check to see if he has a boner. Then you have to wonder if he is thinking about her the next time you make love.
Seriously though, maybe its better to ask her over for dinner.

Think about what exactly ?????

Ask him if he knows her. See what he says.

If they're cheating a very silly way of not keeping it a secret.