inside joke

Sometimes when I'm on the train, or waiting on the platform, I'll randomly think of something that makes me laugh. It's obvious that nothing funny has happened, nor am I talking to anyone at the time. At this point I have to wonder if the people near me
a) take no notice
b) notice, but understand from having done the same thing themselves
c) think I'm a hallucinating psychotic escaped mental patient
d) think they've missed something