Banichi tells Bren that he is best when he is following his professional instinct, and Bren's instincts told him earlier not to trust Cenedi. For that matter, Bren doesn't really seem to trust Banichi 100% and one could argue he has good reason to not be 100% sure of Banichi.

Answering a mysterious summons from Cenedi at this point seems a bit like going down into the scary basement to investigate the noise when you know that there are zombies in the neighborhood, and your only available means of self defense is a hair brush. At least he's not in a bathrobe and high heels. . .