User blog comment:AYET/An awkward situation - Rayleigh at GR12 after Luffy's departure/@comment-3119203-20121022053614

My guess is that after the Straw Hats left, Kizaru got bored and left. That, or he, Kuma, and Sentomaru were recalled to Marineford to prepare for war (or Rayliegh simply escaped). After he got away, the Marines kinda had their hands full for a while. After the finished war clean-up and the admiral battle, they probably sent a search team around Sabaody to see what was going on. Finding no one, they left. The real question is what is going to happen now that he's back.

Or Kuma used his powers to push the memories out of Sentomaru and Kizaru just forgot to mention Rayleigh to Akainu.