You only need one of the four 'clues' available to unlock the option - one of which can only be obtained early in the game and requires a deliberate plan or you'd never have the stats for it in time, I don't know if anyone's even seen it yet.
Esmarai's suspicious are enough for you to want to investigate in the first place (it's possible to not even have the option of investigating!) but not enough to get the fourth option. What clue were you hoping to use?