Dazai noticed that you always wore long sleeves and baggy pants, even when it's hot out. There was one time he insisted on taking off your clothes even if it's just a bit but you then reacted, saying no to his suggestion

You were out on a case and he can't help but be nosy. So he went to your desk and opened the drawers, that's when he saw a razor blade, gauze rolls, alcohol, and others. He caught you, now waiting for your return, sitting crossed legged on the sofa with a stern expression.