Why does he keep tabs on me?

Don't go for another booty call. He is, as you say, emotionally involved.

Apparently only one of you was not emotionally involved. Either that or someone made a lasting impression in that booty call, lol! He definitely wants you to himself. I would ask him about the visit. Say like my roommate said you stopped by, did you need something. Based on his response and reaction, you can gauge how he feels about you. If he is taking it past booty call then I would cut the cords and not do much with him. Be forward with him or he is not going to get the picture. Booty call equals no drama. The minute it has drama then the booty call has been violated.

Maybe he wants more do you.

You are not emotionally involved. He's a little bit unstable. And just so you know stalking is illegal in all 50 states. Who drives to someone's house after they said they weren't home just to see. Especially! When you are NOT in a relationship? Call him out and say that you weren't home your roommate was and didn't hear the doorbell but saw you drive off. Ask him why he would stop by when you told him you weren't home. Id be curious to hear what he has to say.

It sucks, but guys can be possessive even in a situation like that. It could be that he wants you to only be his booty call.