Both Paladins and Traders have a sixth sense, sometimes referred to by players as "Spidey Sense," which allows them to detect certain stealthy actions being performed on them or someone in their group.
Paladins are able to sense many stealthy actions such as mark and advancing from hiding.
Less aggressive actions, such as mark, are harder to detect than more aggressive actions like advancing from hiding. The Paladin will either detect general hostile intent or what the action specifically is. The Clarity spell enhances the sixth sense and allows greater levels of detection, including the ability to see the aggressor's name.
Traders have a more limited version and are only able to detect theft attempts against themselves even if they fail the normal perception check. They cannot detect attempts against someone else. The messaging indicates that you feel your pockets have become lighter and does not indicate who stole from you.