iPhone randomly playing "Night Owl" ringtone?

Since updating to iOS 11, my phone will randomly play the "Night Owl" ringtone. It does this while the phone is locked and not in use, if the phone is not silenced. I have attempted to restart the phone multiple times, and have closed every single app and all possible phone notifications--it still does it, even at all hours of the night. Does anyone have any ideas as to why this might be happening? I do not use the "Night Owl" ringtone for any notifications, so I have no idea why this particular one would be playing.