Bark up the wrong tree

The investigator spent days trying to prove the butler committed the murder, but it turns out he was barking up the wrong tree.

You're not the first man who has made such a mistake, and found he was barking up the wrong tree.

They all went into the house, and left me feeling a precious idiot. I had been barking up the wrong tree this time.

"We want West. He's a cowardly murderer—killed the man who trusted him." . . . "Of course we may be barking up the wrong tree," the officer reflected aloud. "Maybe West isn't within five hundred miles of here."

After three failed marriages I realised that I may have been barking up the wrong tree and should abandon the search for the perfect wife.