User blog comment:B214/Volume 23 discussion blog/@comment-76.175.162.45-20170404042611/@comment-31137559-20170404213135

Grayfia, Rias, and Sona are well aware that they went into that barrier to fight Trihexa. And plus Rossweisse helped work on the barrier to seal Trihexa. Ajuka brought Issei in there alone probably because they told him something meant for Issei's ears alone.

Like I said, it's not like Rias or the others think that Sirzechs and the others went on this thousand-year vacation. What do you suppose they think happened? Most of the major gods, three of the Satans, three of the Great Seraph, and the former leader of the Fallen Angels are gone. It's no big secret that they went inside the barrier to fight Trihexa. Unless everyone thinks they're dead already or died sealing Trihexa permanently.