I'm sorry that Digger has worried you but I do want you to know that this is a VERY common situation. I remember the first time that one of my friends disappeared I was just sick. A week later I saw her returning along the top of the fence like she had never been gone. Since that time I have had this happen many times. She is still out there but never comes home now. I know where she lives so when I see her it's because I go to her place to visit.
You could try banging two nuts together. If they are familiar with that sound it might being him running home... but not necessarily. He might just ignore you. They are a lot like cats in that respect.