I have some situations you may not have considered.

Firstly, What happens to the player that does log out?

Secondly, what about these situations?

• Server goes down as people are fighting?
• Ping kick for one of the players, and they rejoin asap?
• Players internet/power goes out?
• How do you know when you're far enough away, and what if someone has followed you for 200m just to see you log out as they line up a shot?
• How can you tell the difference between someone logging into a strong position to engage, and someone innocently logging in to play without knowledge of the fight, but then engaging in that fight as it's obviously going on?
• What if a player is shot at with a suppressed weapon, genuinely didn't hear the bullet snap, and logs out innocently thinking they are alone?

I too don't like it when players combat log, but it's a very hard thing to prove consistantly.
That and a lot of the "ok will if that happens that's ok" answers lead to abuse. For instance, "Oh, my internet went down" just means I pull the ethernet cable or unplug my PC every time I hear shots.
Or if "the server goes down, that's ok", means an attack on that server may "innocently happen" just as a certain someone makes contact.

The solution you provide will need a lot more fleshing out before it could be considered a solution, but it may be on the right track.