Unless the server has this function disabled, it was just another player being immature. To do this you hold down the CTRL button and left click and drag the mouse on the map to create lines. The same goes for global, group, vehicle, and direct.
If you draw something on the map while on side chat everyone in side chat will be able to see it. I figured you'd want to know so Map drawing is a feature of the game and is dependent upon what chat you are using. To me this suggest someone has accessed the server without permissions or was employing a hack that allowed them to manipulate the map. I'm a noob at Arma but suspect that this shouldn't be happening.
They would then delete what they'd drawn and then draw some more. Forgot your password? I'd like to report unusual activity that suggests either one or more of your servers has been hacked or a rogue admin was having some fun.
