Board Thread:General Discussion/@comment-24518534-20140207202148/@comment-11051098-20140208192424

Renji and Yumichika are the only ones who knows that Ikkaku has Bankai. Well, from Tetsuzaemon's words and Sajin's behavior after Poww was killed, they seem to know too, but that was after Ikkaku used his Bankai to kill Edrad.

Mayuri also mentioned his Bankai when he talked to Ichigo after Tensa Zangetsu was cut in half by Jugo.

I would say that unless his reiryoku was powerful enough to make Yamamoto or others sense that he could have a Bankai, Ikkaku didn't have any seal at the time.