In mystery fiction (not necessarily crime fiction), the important evidence is found by important characters. It doesn't matter how many people are searching, how inexperienced the main character is at mystery solving, or if they are operating out of their field.
They could be off the case, sent home sick, having a day off, late for work, etc. Someone will throw the evidence out the window of a moving car for it to hit them in the face. No matter what, main characters find the evidence.
This trope is understandably found everywhere in mystery fiction, since main characters are meant to drive the plot, yet most authors do not attempt to explain the continuing good fortune of sleuths, both amateur and professional, finding that one piece of evidence that not only seals the mystery up in a neat little package, but also managed to elude everyone else.
- In Detective Conan, minor characters will only find evidence if it's there to throw them off track.
- In Edgar Allan Poe's The Purloined Letter, Detective C. Auguste Dupin finds the titular item because he's the only person clever enough to try to think like the thief does.
- Harry Dresden. But then, there's a very good reason he finds most of it, all things considered.
- As the series goes on, it's more likely for clues to be found by other people (such as CPD, who gradually become more adept at dealing with the supernatural) — but those people don't know what the physical evidence means, so they bring it to him for interpretation.
- Almost any Amateur Sleuth or Little Old Lady Investigates. If they weren't finding the evidence, the story would be about the person who was.
- In Monk and Psych this is justified — they're the only people in their departments who can find the evidence.
- In an early episode of Criminal Minds, the BAU team is assisting a search-and-rescue along with the local authorities and volunteers from the community. The BAU's real goal is to profile the volunteers to see if one of them is the unsub, but still with all of those people looking for clues and the BAU watching the people, not actively searching, it's Elle who finds the note saying the missing girl is going to be sacrificed.
- Ace Attorney, owing to the shocking incompetence of the police force.
- It's also significantly averted,since the cops do find a lot of evidence for the prosecution. Phoenix (or Apollo) is the only one actually looking for evidence that can be used by the defense.
- In the final case of Ace Attorney Investigations: Miles Edgeworth, this is inverted - the final decisive piece of evidence is delivered by an unnamed extra.
- Danganronpa: Trigger Happy Havoc, mostly due to Kirigiri's talent. Other people find evidence, but do not realize that it may be used as evidence until Naegi or Kirigiri learn of it.
- KateModern, probably because the police are being manipulated by the Order.
- Played with in an episode of Justice League. Batman doesn't find any evidence at a crime scene, and suggests minor character The Atom come to find anything he might've missed. However, he apparently doesn't find anything either.