The PDA "look"
This article claims that people with PDA have a certain look in their eyes:
They may have a “look” in their eyes that is indescribable and/or scary to the parents(S) and family members.
As a person with PDA myself, I'm not quite sure what she's talking about. Nevertheless, I do note that I've sometimes met people who seem a lot like me in this regard, and they have a certain vibe coming from them that no other type of person has, so maybe that has something to do with it.