“ A regulation of the Metropolitan Police Department requires the officer who takes an impounded vehicle in charge to search the vehicle thoroughly, to remove all valuables from it, and to attach to the vehicle a property tag listing certain information about the circumstances of the impounding. Pursuant to this regulation, and without a warrant, the arresting officer proceeded to the lot to which petitioner's car had been towed, in order to search the vehicle, to place a property tag on it, to roll up the windows, and to lock the doors. ”

“ Once the door had lawfully been opened, the registration card, with the name of the robbery victim on it, was plainly visible. It has long been settled that objects falling in the plain view of an officer who has a right to be in the position to have that view are subject to seizure and may be introduced in evidence. ”