The police are on a fishing expedition to find any evidence of crime.
fishing expedition
idiomA search for information, especially incriminating information, that is conducted without specific direction or goal, in the hope that something may turn up.
The lawyer's questions felt like a fishing expedition, as he had no proof and was just hoping to find a mistake.
The audit quickly devolved into a fishing expedition, with investigators scouring every financial record in hopes of uncovering some hidden irregularity.