Personally, Zein was more surprised that his background had stayed hidden for this long, considering how much attention and interest the public had toward him.
But it was natural for people to not know. Zein's license had no record prior to his measurement in Althrea, so they couldn't use it to trace his past employment. If they dug a bit deeper, they would find out, through the recommendation letter he used, that he was once part of a Borderland Unit 04-2. But other than that, none.
The Borderland Unit was a place where they accepted just about everyone, which meant, they did not bother to have any record about someone's past. Someone could be a criminal or a maniacal psychopath, they didn't care so long as they got manpower. Crimes were the least of their worry after all.