Smartphones can be a cop's best friend. They are packed with private information like emails, text messages, photos, and calling history. Unsurprisingly, law enforcement agencies now routinely seize and search phones. This occurs at traffic stops, during raids of a target's home or office, and during interrogations and stops at the US border. These searches are frequently conducted without any court order.
Several courts around the country have blessed such searches, and so as a practical matter, if the police seize your phone, there isn't much you can do after the fact to keep your data out of their hands.
However, just because the courts have permitted law enforcement agencies to search seized smartphones, doesn't mean that you—the person whose data is sitting on that device—have any obligation to make it easy for them.