I was around level 10, had just finished the Companions quest line. I was too lazy to purchase Breezehome so I stored all my junks in the barrel in front of Warmaiden (little did I know, your stuff will despawn in 2days.... but that's a story for another day). Anyway, It was late at night when I fast travelled to Whiterun after doing some side quests. All the sudden Dawnguard quest triggered and vampires started to attack the town. Belethor was out that night and all the sudden he pulls Wuuthrad (which was stored in the barrel outside of warmaiden) out of his pocket and start beating down on the vampires. I was so stuned that I forgot to retaliate against the vampires. Fortunately, guards + Belehor = lots of dead vampires. I went to check the barrel after the fight and indeed Wutthrad was gone... From that day on Belethor walks around with Wuuthrad on his back like a boss...
P.S I can't "pics or didn't happen" because I deleted that save file... It was evil!