Also, I wrote as part of Pup the Dog Shaman's storyline that he would have gotten a SIN had he stayed in the legitimate orphanage for a certain length of time, or had been adopted.
As he ran away with his "Sister", the orphanage just collects on his "still being there", and he'll have "Run off" a week or so before the SIN is activated...
Purely my idea, I will admit.