fishing expedition

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).

Allowing the data to suggest hypotheses to be tested. The results of the ensuing tests must be treated with caution. Sometimes called a fishing expedition.

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An attempt to find useful information by asking questions at random. For example, The sales force was told to go on a fishing expedition to find out what they could about the company's competitors. This expression was taken up by lawyers to describe interrogating an adversary in hopes of finding relevant evidence and is now used more broadly still. [c. 1930]

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