How would you respond in this situation?

My boyfriend and I have been together for a couple of months. A couple of days ago, I walked into the cafeteria and I saw him sitting with a girl on his lap. I didn't know how to respond in that situation. We are not a public couple, so she wouldn't know. I am pretty sure it is his friend but I don't know what to do. I never brought it up to him. I didn't even go close to him. I just walked away. Should I address the situation to him? What should I do? I am not really comfortable about this. I don't rub up against over guys. I am not the jealous type but I have to put my foot down when I see this kind of injustice happening, lol. Help?