Nowhere. Tywin wasn’t giving Tyrion a clue, he was dismissing Tysha’s entire existence as unimportant, of no consequence.
Tyrion asking people this is entirely about trauma, hence why he’s also constantly hearing the sound of the crossbow firing. As fans we often don’t think abotu that while there was an 11 year gap between ASOS and ADWD, it’s onlly been about a week for Tyrion between him finding out the truth about Tysha, murdering Shae and his father, and then arriving in Pentos. Especially in Tyrion I, he’s profoundly fucked up over what he did and what was done to him, and the words are a way that he revisits those memories; notably, the more time passes in ADWD, the less frequently he asks that question until he stops altogether.