Zabuza and Haku were sealed away early in the fight, yet Kakashi had Zabuza's Beheader/Executioner Blade later on, so I don't think that that was the case. Granted it was summoned, but it wasn't summoned from the pure world via ET like the swordsmen, it was summoned from the impure world, probably where Suigetsu lost it; thus I doubt it simply turned to dust or sealed. That doesn't make sense to me.
Why I think Kakashi might not have it... He had great respect for Zabuza and Haku after fighting them (Kabuto's Edo Tensei'ing them is what got his blood boiling so quickly that he went on a rampage so quickly), and I wouldn't be surprised if he used it as a grave marker again.
That sword appears to have a very strong emotional valence for Kakashi, so whatever he did with it was not a lightly made decision. It's only speculation on my part, but it makes sense for him to use it as a grave marker, or bury it, something along those lines and he's done it before. It may even feel tainted to him, or he may feel unworthy of it for whatever reason. He was a badass though... :kakashi:
Peace
