I think this information will be useful for some people. Especially now, when high karma bonuses events are present.
I don't know scince when, but markers on maps are now synchronized for all players. So you can take a map with you, and use it as some kind of a notepad on which you can write texts and messages for other players.
Place map inside a container, for exaple – dropped backpack on a floor. So several players can examine this backpack and everyone can open map at the same time. Right click on map, choose "view" option to open map. Right click on map – "create marker".
Every map can store 100 markers. Every marker can hold up to 75 symbols. When you hover your mouse over a marker – it shows its text.
New markers from players will appear on a map in real time, if several players will open one map at the same time.
I hope more people will use this thing to communicate. I played for a long time today, and only one person figured out why am I dropping backpack with a map inside. Only because I spent some ammo to write "open map" with a bullet holes on a wall! xD