Did Danzo neglect to place his curse mark on Kabuto's tongue? It seems even more important considering he was Danzo's spy and could have been captured and tortured for information. As far as I'm aware, all other members of "Root" had the curse mark except maybe Orochimaru, so perhaps Danzo didn't have curse mark jutsu at that point in time, but I thought I'd ask in case one of you fine manga readers had the answer.
You are right, it was negligence from Danzo's side, not having cursed Kabuto. That's why he brainwashes Nonō and sends her to kill him. When she fails, Orochimaru was sent to finish him off.