The Collaborative International Dictionary
fishing expedition
fishing expedition \fishing expedition\ n. An investigation searching for evidence of wrongdoing, with no credible evidence of such wrongdoing available at the outset of the investigation, and often without specifying in advance the wrongdoing to be proven. [metaphorical]
Wiktionary
fishing expedition
n. (context idiomatic legal US informal pejorative English) A non-specific search for information, especially incriminating information.