Why would a man walk past you then look back at you and smile?

Why would a man walk past you then look back at you and smile?

I've drawn the floorplan for reference, I'm the red dot and my colleague is the yellow dot.

I (red) came out of a room and into the corridor and when I walked out my colleague was at the yellow dot, he was on the phone and when I got close behind him he turned around, full body, and smiled. Why?

Why would a man walk past you then look back at you and smile?
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