YA novel where main character outs secret agent once he sees handmade shoes

[Sarah]

It’s one specific scene in a YA novel where our main character is suspicious when he sees the shoes of a detective or agent who is trying to help him because the shoes look almost handmade. I think he notices this through the door. The detective or agent shows his card through the door I think? He had a name like Agent D or something like that. I cannot for the life of me remember what this is from but I read it as a middle schooler about 6 years ago

[Rainbowheart]

Was the boy a teenager or a younger child?

Do you remember why the agent was trying to help him?