He was mentioned to have a new assignment dealing with the Phantomhives so it's possible. Also he's one of Yana's favorite, she's definably bringing him back.
I agree with vamp_grl_123.
Ronald mentioned an assignment Grell had dealing with the Phantomhive household, so... there's just not much more to it than that lol. It's not unlikely.
I agree with emiliux123. He's there with Ronald Knoxx collecting souls from people eaten by zombies! Yay!!! Anyway you will be looking forward to the point when Grell sees Sebastian is there! Like Grell says, the purest form of love is a death match! XD
