It is not that they didn't cooperate, but rather the police didn't think they where cooperating, at least not fully.
The problem the police has is that can't believe people when they say they don't have something. Imagine if someone killed somebody with a gun then gave the gun to some other-person, and when the police asked that other-person if they had the gun they said no. The police acting on a reasonable belief that they might have the gun, could get a warrant permitting a search for and the seizer of that gun, they wouldn't need to get a warrant if that other-person just gave them that gun. Of course there is still a problem that the other-person might never of had the gun but the police shouldn't just take them on their word.