User blog comment:B214/Kuoh Academy students finding out that Issei stays with the ORC girls and is dating Rias./@comment-4547267-20131217014928/@comment-1359322-20131217021440

Nothing, Sirzechs knew that Grayfia was drunk at that time, so he'll know that it's another one of those Grayfia's "troublemaker" time that she does when she's drunk. After all she was just teasing Issei until Rias and the girls enter and she immediately switch to teasing the girls instead.