They might have just farted (silent but deadly)
Why would someone appear suddenly smiling for no reason while talking?
I smile often about funny, things that have happened in the past.
I do that sometimes c: If I'm telling a story, and I think of why I'm even telling it in the first place... It makes me smile :D
It can so many varied reasons, it happens for me when i remember something funny i watched, something funny i've been told or even having people that i care about back in my mind.
Gosh I can't stand it when people do that. It's as though they smile at you because they're not sure you're getting what they say so they smile because they're thinking what a twit you are.
In short I feel it's patronizing.
Maybe they just remembered something funny...I do that a lot :)