They refuse to share the secrets of their race to others even when on trial for their lives. Some people still believe they are were-creatures and that they turn invisible during a full moon as most of their attacks occur near to a full moon. Other traditions say they are an association of criminals with stealth technology from the Old Ones.
Almost all Not-Men who are caught for their crimes escape, even when under heavy guard, and in some circumstances they seem to have slipped through the bars of their jail cell leading to some people believe they have shape-shifting capabilities.
In the Marvel Comics, Not-Men have been seen to take on Human forms when not invisible, so it could be possible that their two main forms are invisible and human.