I'm aware there is an 'S' prefix being added to Scottish resident taxpayers tax codes, but asking around up here, that hasn't happened for everyone.....
Presumably because their main residence is not in Scotland (or HMRC does not think their main residence is in Scotland)? Or perhaps they have not had a new tax code issued yet? I'm comfortably sure you have the legal right to set your own tax domicile assuming you have a proper address to register it to. A single person not owning a home for example, could leave it at their parents house. Lots of opportunities for those that care enough. I'm planning on switching mine in May; I'll report how it goes back here.