fishing expedition

from WordNet (r) 3.0 (2006)

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

from The Collaborative International Dictionary of English v.0.48

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]
   [PJC]