fishing expedition

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an investigation undertaken in the hope (but not the stated purpose) of discovering information