If professor does not smile at you?

it is strange.. cuz there was one time I was sitting at a bench with my friend, then as my back was facing the corridor, when my professor and his colleagues ped by I thought I saw him, but he walked quite fast and at first when I looked back at him, he didnt look back so i thought he did not notice me either. But then he noticed me the second time i glanced back so he just waved to me with no smile and stare at me (walking backwards for quite some distance) until he almost knocked against another bench. so I felt weird? also in cl when i looked at him, he doesnt make much eye contact, and in lecture when i sensed he is looking at me i looked up and he looked up (as in to ceiling)... with no smile. why is it so?