Use the [Frosthound's Collar] at the Abandoned Camp to track down whoever took Brann's possessions. When you recover them, use the communication device and speak to whoever you can contact.
The tone of Brann's note suggests he meant to bury something with the note, maybe a pack of some kind. You're sure this was all you found?
Ok, I'm going to have you take one of these frosthounds with you. The frostborn use them for tracking. Bring it to the destroyed camp and let it get Brann's scent.
It should at least lead you to the location of those missing possessions, and if we're lucky, Brann himself.
Completion
You... you're not a member of the Explorers' League, at least not one I remember. But, if you've recovered the communicator, they must've sent you. Good, that's almost what I was hoping for.
Notes
Use the collar at the camp (after clearing it out), then follow the trail. Hit 1 to net pursuers and 2 to lay down a frost trap if they start to overwhelm. The frosthound will lead to Frosthowl Cavern at the southeast end of The Foot Steppes, where Tracker Thulin awaits. Kill him to loot [Brann's Communicator].