Uh-oh.

怪しい (Ayashii)… how could she have known indeed…? I wonder if someone tipped her off…

This is probably the prequel to this.

TL notes:

  1. Translation adapted from Danbooru

Artist Commentary: How did you imagine they wouldn’t know, Sanae?

How, indeed…

Source: “Sanae has been found out.” by Suwaneko (NSFW)