fishing expedition

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An open-ended inquiry or investigation, often undertaken on the pretext of a minor or unrelated matter, whose real purpose is to uncover embarrassing or damaging information, as about a political opponent: "[enabled] prosecutors to expand what started out as an investigation of ... [a] land deal into a fishing expedition for intimate details of his daily—and nightly—life” ( Margaret Carlson).

Wiktionary

1. n. idiomatic, law, US, informal, pejorative A non-specific search for information, especially incriminating information.

GNU Webster's 1913

1. n. metaphorical 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.

WordNet 3.0

1. n. an investigation undertaken in the hope (but not the stated purpose) of discovering information