That’s correct, fetishes can only be revealed once per day at the moment.
An Undiscovered Fetish means that your characters either have a (or possible a couple) Dominant, Submission, Sadistic, or Masochistic traits. The best way to “discover” that trait is to talk to them about how they feel about inflicting pain, feeling pain, taking control, or giving up control. When you find their fetish that has acceptable/mindblowing, it will change from Undiscovered to the actual trait’s name.