When I was in college, I was dating a girl who was taking a philosophy class that was particularly interesting to me. One day we were hanging out before her class and she was telling me about their discussions, and it covered some ideas I really enjoyed discussing. Since I was in computer science, I never got the opportunity to take a lot of humanities like Philosophy and so I mentioned that I wished I could take this class. She had to go to class and just said "you can come with me if you want".
I went with her and just sat down. I took notes, I participated in discussion, and ended up going back for several weeks. I eventually stopped going as they moved onto another chapter. I popped back in a few weeks later and the only thing the teacher said when they saw me come into the lecture hall was "hey it's good to see you again". The prof knew I existed but either didn't realize or didn't care that I wasn't actually on the class roll.