In the Banquet Hall, Black Maria has Sanji tied up in her webs, revealing that he defeated all of her male subordinates but never hurt her female subordinates, reasoning that as the reason he lost to her.
She then demands he call for Nico Robin, telling him to speak to one of her subordinates to echo his voice throughout the castle and in return, she will release him. Sanji asks her what she wants with Robin, and Black Maria answers that they will capture her, mutilate her to incapacitate her, and once they are finally finished with her, they will kill her.
I think it is now clear where Nico Robin is. And that is right in the very room that Sanji is being crucified in! Nico Robin is definitely the person behind that eye mask.
Notice the murmurs coming out of the womans mouth (the dots) and then Sanji thinking about it in his head with a question mark…? This type of emphasis towards a random character who should have no relations to eachother would be useless and confusing, but things change if this is Robin. Somehow, maybe just from the mutter itself, Sanji could have been able to register that the person he is talking to is Robin, and in fact even looks at her! This would help resolve the current Sanji situation faster and get a Black Maria vs Nico Robin fight getting even sooner.
I wonder if that was also Nico Robin in the room with CP0. If you think about it, Nico Robin’s intelligence gathering with her devil fruit would work extremely well with the surveillance/broadcasting these eye papers seem to have. Definitely it’s in her character to take advantage of something like that.