Here's a couple of links on AO3. I haven't read either of them, and Cas isn't present, unfortunately.

Cry Wolf (http://archiveofourown.org/works/2968/chapters/3683)
47,447 words. It's a Sam/Dean pairing, and I'm not sure if either of them are sentinels/guides.

Twenty-Four Hours (http://archiveofourown.org/works/2704)
10,381 words. Gen fic. Again, not sure if either of the boys are sentinels/guides.