Fox kits will often appear unsupervised for long periods while their parents are out hunting for food. They will play like puppies around the den site until the parents decide they’re old enough to go on hunting trips. Then they will suddenly disappear. Observe the kits from a distance; if they seem energetic and healthy, leave them alone. If they appear sickly or weak, or if you have reason to believe both parents are dead, contact a licensed wildlife rehabilitator. (Excerpted from the Humane Society)
It is not uncommon to see an adult fox that is missing patches of fur. This is often a sign of mange, a parasitic disease that is often easily addressed with the right knowledge and medication. Please contact a licensed wildlife rehabilitator.