That's is odd. If you are both high enough lvl to grey everything in the zone, I suggest a sweep pattern. Each of you line up going north, standing within visual distance of each other, and head in that direction. When you reach a wall, move a little bit east, then repeat going south. rinse and repeat (you get the idea) until you cover the whole valley.
Make sure you look behind rocks and such also. Some of them are a little hard to spot. Also make sure you save the sigil in the tower for last.
Also, make sure you are in the correct zone instance. There are multiple instances for Firemyst gully, and only The Cleansing of Firemyst gully has the sigils. If you see tree stumps that con as mobs and dwarves that are ghosts, you are in the wrong one. It may be that your lvl prohibits this instance from opening up. I'm not sure, but I think the lvl limit for The Cleansing is 25. I could be wrong.