I can see no reason NOT to require a warrant. I mean they’ve arrested somebody so presumably they already have probably cause, so getting a warrant if there is a possibility that there is evidence on the phone related to the crime that they have arrested the person for should not be difficult. And once they’ve booked the phone as property, any evidence on it is presumably secure. So the ONLY reason that I can see to allow warrantless searches of the phone is to go on a “fishing expedition,” for the possibility of UNRELATED criminal evidence. Say, they arrested you for a bar fight, but think that you MIGHT be a drug dealer.
