So let's do this scenario: you're a coworker of mine who I don't work with directly but we see each other on a daily basis. We've bumped into each other in the bathroom, the elevator and once sat together at lunch because there were no other empty tables available. One day after work, I'm walking to my car and I see you walking to your car. I say "Good Night!" and you look at me and turn away without saying a word. I think (and I'm going to be bluntly honest here):
1. Did you hear me?
2. I try to remember all of our transactions together and if I did/said something to cause offense. At this point, I do think we've had enough transactions together to not be strangers so if I say good night I think it's fair for me to expect you to say good night back. I don't expect a long intimate conversation, but I do expect my existence to be acknowledged, especially when I speak to you directly.
3. I try again. If I see you at some point in the near future I'll purposely say hello or start a small conversation and watch your response. I'll say something like, "hey, I saw you going to you car on such and such night. I hope your commute isn't as bad as mine!" I do this to see if you in fact did hear me and also to see if I can pick up any animosity coming from you towards me. If I sense nothing then I just come out with it, "yeah I said hello but maybe you didn't hear me?" You can either confirm or deny not hearing me.
But really, I know that you saw me because we made eye contact. If you say you didn't see me then I know you're a liar and I don't have anymore dealings with you. I walk past you and ignore you and you're non-existent to me. With me, you wouldn't have to worry about me asking what's wrong cause I wouldn't care.