The best things to say when you don't know someone's name and they seem to know yours are . . .
"Hey, big guy!"
"Hey there, chief!"
"Good to see you!"
Any of those will do, but when you work with kids like I do, and every damn kid seems to know Dr. Ken's name, you do like this:
Kid: Hi, Dr. Kenneth Noisewater!
Dr. Ken: There he is!
The expression "There he is!" implies that you have been looking for him or just looking forward to seeing him, making the guy feel special, even though usually you're pretty sure you've never met him. It does wonders, especially if you deliver it with a smile and some enthusiasm.
What's your standby phrase when you don't know who in the heck someone is?