But
Spoiler!
Because he was really cautious and skeptical, and the rumors that he's a cunning/crazy old man, he ignored those clues. Livia & Aleric were really worried about him but Tessetan is really deep into his thoughts, ignored them and stumbled his way up to his room.
During his time, he's properly processing the facts like, how powerful Tie is and how she really is in danger if the Order caught her because she even stopped his mana rampage which his mother, the strongest Saintess, failed to do. And that Agavert's efforts into hiding her identity made so much sense. Then through his window, he saw how the headmaid lovingly put Tie to bed.
During his time, he's properly processing the facts like, how powerful Tie is and how she really is in danger if the Order caught her because she even stopped his mana rampage which his mother, the strongest Saintess, failed to do. And that Agavert's efforts into hiding her identity made so much sense. Then through his window, he saw how the headmaid lovingly put Tie to bed.
He cursed realizing again that Tie is really just a child. He even recalled her line 'i came to find my dad' and he remembered the moment at the port where he saw Tie crying calling him Dad.
He thinks it's impossible she's his child because he haven't properly slept with a woman. So he thought Tie mistook him for her dad. Like maybe he resembled him. So he thought that is the reason why the child likes him and why she stare at his face and sticks to side at any chance possible. That night, he sat on the floor curled up and didn't sleep a wink.
I like to post spoilers for the stories I find interesting because I don't have much time to read novel due to busy schedule
you will find me in most romantic historical manhwa comment sections 
you will find me in most romantic historical manhwa comment sections