User blog comment:LexFacie/Thoughts about Bankais./@comment-11051098-20140710161106

First of all, all Royal Guard members are former captains in the Gotei 13. Kenpachi Zaraki is the only one who's become a captain without having a Bankai, so all 5 Royal Guards has a Bankai.

Although there's no proof of it yet, I do believe they all know who Yhwach is, even Tenjirô. Yhwach is the kind of great threat everyone in the Soul Society should know about, even if no one expected him to ever return. Tenjirõ is probably just the confident type, thinking that his skills as a member of the Royal Guard makes him capable of defeating even someone as powerful as Yhwach.

About the Bankai:

The old Zangetsu appears to have been created by Ichigo's Quincy powers, the "Old Man". It seems to have acted in the manner of a real Zanpakutô, having a Bankai form and that, but it had no Asauchi as a base. This is probably the reason it could not be stolen, as it was never a "real" Zanpakutô and thus not a real Bankai.

Further, Ichigo's Shinigami powers are fused with his inner Hollow, so even if it had been real, I doubt it could have been stolen. That also means it's probably impossible for the Asauchi-based Zangetsu Ichigo now possesses, to be stolen.