When your dog is outside a safe zone, their Fi Collar connects to your mobile device. See: How Can I See What My Dog’s Fi Collar Is Connected To?. If the Bluetooth connection between your dog's collar and the Fi App breaks, you'll receive a notification indicating your dog is without an owner.
This can most commonly happen at a dog park, where your dog may run far away from you to play, but yet they are safe. If this is a familiar park your dog likes to visit, we recommend making your dog park a Safe Zone to prevent these alerts. See: How Do I Setup and Edit a Safe Zone?