Was he being just witty and friendly or was he flirting?

We're coworkers. Today, as his shift ended, he opened the office door to leave, but just before disappearing behind it, he glanced in my direction. I caught it. Then, he pushed the door open again and waited until I looked up at him. Our eyes locked for a solid three seconds.
Then, he traced a smile on his face, as if he's telling me to smile. I responded by pulling the corners of my lips with my fingers. He laughed before finally leaving.
Was he flirting? Or am I overthinking it?
PS: I wasn’t even that sad.
Was he being just witty and friendly or was he flirting?
Was he being just witty and friendly or was he flirting?
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