??? I'm on Chasing Echoes, and I show up to complete Hide and Seek and Ancient Technology...and... all Dawnguard npcs are hostile. I have no bounty. I'm a werewolf, but they've never seen me transform or in wolf form... Don't really know what to do now..

Anyone else have this issue?

I haven't had that issue specifically, but I have had to reload once or twice from NPCs turning hostile mid-combat. Presume it's just my eyesight and tendency to swing a sword rather wildly, though.

I had the exact same problem. I was able to fix it on my own though. I made a thread about it that should help give some insight on why you're having this problem and how to fix it.

http://www.gamesas.com/topic/1390415-everything-suddenly-become-hostile-in-your-game-fix-here/

I had a problem like that after I had finished the quest line, but I wasn't a werewolf or a vampire. I had actually just had my vampirism cured, and I returned to Fort Dawnguard to pick up some steel bolts and a quest from Gunmar when I noticed everyone was trying to kill me. I loaded an autosave, left Fort Dawnguard and waited somewhere for 3 days. When I came back, no one was hostile, so I'd recommend trying that if you haven't already.