Sometimes I will look at my map, I can see other players. Sometimes they are blue and sometimes they are orange. Other times, I know someone is near me, but there is only me on my map.
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According to this forum post, the orange marks on your map are "uncomfirmed" zombies or players.
Confirming the zombie/player will turn the icon blue and give it the appropriate label (Civilian for Zombies, and Survivor/Sniper/other variation dependent on player skin choices for humans). In order to confirm the blip on your map you must be able to actually see what it is the map is pinging.
This setting is generally frowned upon, as it often shows vehicles and camps as well (making remaining hidden very difficult). It is apparently an option the server administrator can enable or disable.