@okabekudo
He's often been careless, even from the start. He gets very cocky sometimes, and lets his guard down. His plausible deniability with Sera has lasted a lot longer than he could keep it up with Hattori, after all. he just isn't normally around detectives smart enough to put two and two together, and willing to accept that he could have been reverted in age. He's even had Ran believe him to be Kudo on multiple occasions, needing major help to cover things back up and throw her off the trail.
Should he be more careful of his words around Sera? If he doesn't want her to know, sure, but he has been careless multiple times in the past, even from early on. (Though, to be honest, I'm not entirely sure why he still wants to deny what she seems convinced of, rather than just admitting it to her at this point, especially since he knows her family, and knows the condition of her mother, so he knows she is opposed to the BO. Better to be honest with her, in private, and get her to agree to help him keep his secret, than risk her exposing it to Ran before he feels it to be the right time.)