My take on the {"api":"faces","found":{"success":true,"predictions":[{"confidence":0,"userid":"unknown", is that DS THINKS it is a face, but that you have not added THAT face to the "faces" DB (image set).
Normally if mine does NOT find something it THINKS is a face, I get this {"api":"faces","found":{"success":true,"predictions":[]}},
If it finds something it THINKS is a face, AND matches it to something in the faces DB (image set) I see something like {"api":"faces","found":{"success":true,"predictions":[{"confidence":65.37463,"userid":"Steve",
Not sure that helps much, but thought I would pass it along.