Well, there's the fact that we don't know what he did for twelve years, but he obviously survived (and eventually had a place to live where Sirius could lie low, post-Hogwarts, at which point I'm thinking he's still on Dumbledore's payroll), though he says he couldn't find paid work as an adult.

It is possible he had a string of jobs and got run out of each one as his secret was discovered, but the question is, was he registered previously? If so, is the registry public, or only available to the govt or to potential employers? Because apparently, the general wizarding populace didn't know he was a werewolf before Snape let it 'slip', or they knew and didn't care that he was teaching their children (which I find hard to believe, given how we're told werewolves basically have no rights, and look at Ron's reaction to him in the Shrieking Shack).

So he could have gotten jobs, and then when his absences were noted, someone checked him out on the registry, or he wasn't registered until he left hogwarts, when he had to, because he was outed.

Or it's 1am and I'm talking out my hat... *g*