He's sitting right in front of you ~Bungo Stray Dogs

Red pen ink

Following his precise schedule, Kunikida was right on time to work. He inhaled deeply and opened the door, as he heard the door shut he let out the breath.
"A letter was sent here last night, take a loot at it. It's on the counter." President Fukuzawa greeted him.
"Yes sir." Kunikida found himself fixing his posture and lowering his head in respect. Fukuzawa nodded at him and continued into his office.

Kunikida delicately picked up and examined the black envelope. The top was already torn open and there was no return address, nor a stamp on it. He took out the 5 folded paper within it and began to read. He set the papers down and looked back in the envelope. "Hmm." he said to himself. There was a pale yellow sticky note in it that he had missed before. He plucked out the note and read the sloppy, red pen ink.
"Enclosed within this envelope is a check for 1.28million yen and all of the files and information on a missing child case from 2015. Only once Detectives Osamu Dazai and Doppo Kunikida solve the case, can the check be cashed." Kunikida read it aloud to himself. He searched through the envelope again, no check. He set it down and walked towards the President's Office. Once he was told to come in, he read the sticky note out loud.
"The check is in my possession for the time being." Fukuzawa assumed the reason for his visit. "Thank you, sir. Also, why request Dazai and I to solve the case? Ranpo is much more competent in solving cold cases." Kunikida asked. "I could not tell you. I don't even know who sent it." Fukuzawa told him.

Kunikida walked back to his desk and began to analyse the information on the missing child. The victim was a teenage, malnourished, boy who was never identified and was kidnapped from Yokohama Municipal Citizen's Hospital 4 days after he arrived.
Kunikida traced his finger down the page.
Months after the kidnapping, the victim had been spotted in public on numerous occasions. Bystanders had tried to interfere and contact law enforcements, strangely, all attempts had been unsuccessful.

The main door to the detective agency swung open. "You're late." Kunikida pushed up his glasses and turned his head to Dazai, who had just walked into work.

A smile formed on Dazai's face. "I'm so sorry, Kunikida! I slept through my alarm and I couldn't find my keys this morning! My deepest, and most sincere apologies. How will you ever forgive me!" Dazai bowed and swiped his hands under his eyes to add emphasis.

Kunikida was fuming, instead of lashing out, he slid the envelope to Dazai. "We've got a job." he gestured to it. "Oh?" Dazai picked up the sticky note first. He blinked once at it and dropped it back on the table. "Kunikida, do you know who sent this?" He asked. "No, why?" He looked up. Dazai exhaled through his nose. "No reason. Anyways, what kind of case is this?" he changed the subject. "A kidnapping, from years ago." Kunikida told him. "Oh, what happened?" Dazai asked. Kunikida moved the papers towards him. "Why don't you find out." he suggested.

He looked at the papers, reading each and every word, noting the punctuation and tone of the document. He committed every date, time, and location to memory. "Have you finished reading yet?" Kunikida looked at the wide-eyed man who had flipped between the papers multiple times already. "What?" he looked up. "What's so interesting about the case? You keep re-reading it." Kunikida pointed it out. "Oh, was I?" Dazai set the papers down.

"I don't think you should be assigned to this case."