It's because either you, or they, are further ahead in some quests. If someone completes a quest the area can look completely different to them and you. You might see no mobs at all whereas they see lots!

I noticed this with a friend of mine, because I'd grouped with him and went to help him in an area I'd already completed a quest in - I saw nothing at all in the way of mobs and I too could only see the "chevron" marking where he was supposed to be. I couldn't do a thing to help him, but when we left the area my pal "reappeared".