I was recently reading through Cyberpunk 2020's Wildside. The definitive book for Fixers.

One of the things in it is SINs. Basically between pages 61 and 65 it tells you how SINs operate. It also makes fake SINs more expensive and brings in such ideas as Skeletons that only work for a while and the concept of being zeroed. As in the information about you is zip, nada, none.

While the majority of Shadowrunners do not have a SIN it does not mean they are ghosts in the system. Instead they are simply in many different databases under many different aliases.

With biometric data being more common it would also indicate an ease in being located easier through the myriad of cameras that every urban environment posesses. Again something also covered in Wildside, except maybe the biometric data.

I see SINs in the BBB obliquely referring to the article in Wildside.

Wildside is worth the buy. Not only for its description of SINs but also for its clear and concise rendition of what a black and gray market economy is.