I think it as simple flirting borderline friendly but I'm not always sure if the guy is just being friendly or showing interest...thats the thing with smiling, can't be so sure! smiling is sort of like common courtesy when you speak with someone or you're holding a conversation..sort of...its not necessary but you want seem friendly and nice right ha ha ha so if you're trying to signal to a girl that you're interested...you're gonna have to do more than that but smiling doesn't hurt...the whole world should smile more often lol sorry went off topic there :)
It depends in what way you're smiling. Smiling can definitely be a form of flirting, but if I see someone I know and I smile at them, does it mean I'm flirting? No it doesn't. A smile can also be a friendly hello or a sign of acknowledgment.
Haha, oh goodness no. There are smirks which can be pretty flirty, but smiling in itself is innocent. Unfortunately, a lot of guys take it as something more and than when they try to act on it they get rejected. Smiling for me just means that you're friendly. If I smiled at someone and they smiled back I would take that as an invitation to approach them and nothing more.