a fishing expedition
An attempt to discover information without knowing what it may be. The term implies that the search does not have a specific subject but is rather performed in the hopes that some new information will be found that will be beneficial to the searcher. Primarily heard in US. The defense attorney objected to the vague line of questioning, accusing the prosecutor of engaging in a fishing expedition.
a fishing expeditionmainly AMERICAN
If you are on a fishing expedition, you are trying to find out facts about something, often secretly. I know why you're here. You're on a fishing expedition. You're hunting for material. He was asked whether Wilkey was engaged in a fishing expedition aimed at politically embarrassing the House.