@emilygorcenski
While you can not be forced to provide a password without a warrant, your phone actually doesn't care. It stores the encryption key in memory in order to facilitate features such as always on notifications, and the unlock screen merely provides one way to access it. We already know that PDs use automatic unlocking devices that bypass the unlock screen to get at phones that come in powered.
The only way to prevent someone from being able to get into your phone is to turn it off.