How could the Water Shuriken be on the bridge when Pikachu found it?

Shortly after Detective Pikachu left Tim, after realising he freed Mewtwo, he walks on the bridge where the Greninjas attacked Harry. Pikachu stumbles upon a Water Shuriken, which he uses to deduce the events behind the attack.

How was it possible that the shuriken was actually there for Pikachu to find? There are two big reasons why I feel it shouldn't have been there at all:

1. Greninja makes Shurikens out of water. They should have dissipated into the water and soon evaporated after hitting the car or bridge. It appeared that it was made of glass in the film, which Greninja cannot make. Greninja can use Ice-type attacks, though an Ice Shuriken would have still melted.
2. The police had investigated the crime scene. It is extremely unlikely that the shuriken would have been missed during the investigation by multiple police officers and perhaps also firemen and detectives. There could be the counter-argument that the police never actually investigated the scene and just used the footage of the holograms as their investigation, but this is counteracted by the fact that the police confirmed that the body was not found - this would have required an investigation of the crime scene.