How is it determined when you can see a player in red?

Does anyone have the logic or explanation for when you can see a red player to shoot at in game?

I seem to constantly run into teams who are firing mortars from all over the place, but I have no red players even on my screen to shoot back at. I lost a game today when my entire team was killed within 1 minute, and I was the highest total with under 1k damage - which was by randomly firing my mortar and getting a lucky hit. It didn't seem like any of us could even find a red player to shoot at (we were Mk4s, mostly).

So, how is it determined when red players appear on your screen or map?

Thanks.

 

Basically, what one player sees, the whole team sees. So if one enemy gets in range (range is quite short, I think 25-30), and he sees your team, then all the far away enemies can also see your team, while you cannot see them, because none of you got close enough to scout them. Because many weapons have longer range than the scouting range, it means you can be hit by invisible boats.

 

Also people take blind shots at start of game, anticipating where you will be. It works better in some maps than others.