Only one person gets the warning at a time, so if someone else took the mission before you and is "waiting" (not necessarily at the right place) you need to wait for them...
As for where it is, you should be standing on the top of a wooded hill just south of the karavan temple, facing west, at the correct time (which I can't remember what time it is) and will lead you down the slope, past some gingo/ragus to a little platform below the main cliffline (at the 'edge of the map').
gl and hope it helps
