Why would someone mouth words at me without sound like they know me, even though we've never met before?

I was running an errand with my roommate. We were standing outside when my roommate said, "He said hi," so I smiled back at the guy. He didn’t say a word, but he kept staring at me. He moved his mouth like he was saying a few sentences, but I couldn’t hear a single sound. There weren’t many people around, and it wasn’t very loud outside. It felt like he knew me, but just couldn’t speak. He wasn’t on the phone; he was having a silent conversation with me.

I tried to read his lips, pretending to be a lip reader. I ended up giving him a confused, awkward smile and kind of said with my expression, “Have a good day.” I’ve never met him before. My roommate noticed him doing all of this, and I was just thinking, “Okay… who are you, and what are you trying to say besides just ‘hi’?”

This is a new experience for me, so I’m here hoping you can help me understand what might’ve been going on.
Why would someone mouth words at me without sound like they know me, even though we've never met before?
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