I know this question may be asked alot. But I have it. I dismissed Stenvar as my follower of Farkas, for a small moment. So as normal. Stenvar leaves. Well I wen't back to the Hall in Windhelm. No sign of him. I've waited several hours and traveled back. No luck. I wouldn't be so worried about it being Stenvar. But he had one of my Dragon Priest Masks.

I think you have to wait several days for them to come back.

No luck. I've done 8 quests and slept 13 times lol. Is it possible he died on the way to Windhelm?

He's not essential, so it's possible for him to die if he's in the same cell as you at some point as he is travelling home. It's uncommon for followers to die, though. He is also confirmed as having the follower bug where they get stuck in a kneeling position and 'crawl' everywhere, which means he could be taking a really long time to return to Windhelm.

Oh....dang. I tried fast traveling to common dangerous places en route to Windhelm. I personally don't care about him. I just want my mask back.

On PC you can open the console and type: player.moveto 000B998C close console.

Be sure to make a hard save before doing this - and I don't really recommend it because the moveto command has a lot of bad press about breaking games way into the future. This will move you to him if he's alive. And maybe send you to death's waiting room if he's dead...