It looks like a particle effected, but It was there before anyone fired a weapon. I went back to watch the replay and it still there too. In Fact, The red dot was showing up in all my replays, until I refreshed the webpage anyways.

This has happened to me before too.

From what I remember, it's created by your drone.
For me, it also appeared in all matches until I refreshed the game.

I've had this problem in both red, and blue (whilst using the red-firing drne, and blue-firing drone obviously)