User blog comment:B214/Question and Answers 6/@comment-26293136-20161107124658/@comment-10330822-20161108102948

Typically their memories are erased or altered, since devils at least have that ability. In the past, they might have been killed. When Kiryuu found out, it seems they just swore her to secrecy.

Besides, the whole proposal that such things existing would probably be seen as madness to normal people, they'd probably think the person had gone crazy. So, even if they do miss people from time to time, for the most part its harmless if it doesn't happen too often.

For example, even if Kiryuu had blabbed to other people, chances are they wouldn't have been taken seriously anyway, and Kiryuu would have been exposed to the devils.