Why would a guy (I’m not dating) give a guy a stern look?

I think this guy has been showing some signs of attraction maybe subtle

But I remember one day I was talking to a guy (I didn’t know) just a brief chat. While talking I saw him give him a stern look while standing 6ft away.

It was a stern and quick look…

It was strange

Why would a guy (I’m not dating) give a guy a stern look?
Post Opinion