This just came to me when thinking, because I had just finished reading the manga and watching the anime as far as I can of Detective Conan/Case Closed.
You must know episode 304 in the Detective Conan series where Officer Takagi and Conan got suck in the elevator-shift and mercury level bomb was one top of the elevator. Well I got to think if Takagi put the time in to trying to found out how Conan know a lot of thing that some adult and most kids his age don't even know. He probably would find out Conan's secret.
Title
Late evening in Tokyo Police HQ, Takagi and Sato are doing some filing. "Hey Takagi why are you in daze?" Sato questions him.
"Huh. Oh, sorry I'm just thinking okay. " He said in a neutral tone, as he began filing again. He was just about ready to put the files of crimes that had happen earlier that day in the cabinet when a file hit him square on the head.
"What are you thinking about? Come tell me!" Sato demanded with an angry look that made he flinch.
"That Conan kid that is living at Mr. Moore's place." He said with confusion in his voice. "Yeah. What about Conan?" Sato questions him. "He doesn't act like a little kid to me. In the elevator-shift his body language changed completely." "What are you saying?" "He just doesn't sit right with me, I mean his attitude." "Okay, just talk to him like I don't like you talk with everyone else and sit down and have a little chat with Conan." "He just scares me a little right now." Takagi whispers to his partner. "Why he just a child?" "He just doesn't feel right." "Sato, what doesn't feel right for Takagi?" Shiratori asks as he comes towards them. "Takagi think that Conan isn't normal." "Takagi, why doesn't Conan feel that way to you?" "You two weren't in the elevator shift when his body language changed in front of me everything changed except for the small body and pitch of his voice." "Just talk to him it will make you feel better plus a week before that happen you told me you kind of like the little fellow." "Fine I'll talk to him one more thing I had a moment where I was being kind of stupid and asked 'who are you' to Conan and he answered 'I'll tell you in the afterlife." "Well talk to in a graveyard that way you can get the answer because some people believe it is the gateway to the afterlife… Oh one more thing, you got to tell us what you and Conan talk about."