A general reference to any individual within any setting who has earned a reputation for belligerence in public places for no apparent reason.
The remark generally implies a weariness of the situation and identifies that the person making the remark would like to immediately remove his or her self from the circumstance.
The remark does not absolutely pronounce that one has had personal experience with such individual but rather is a subtle way of identifying one's own general weariness about any potential interaction with such 'type'.
The primary difference between a Karen and "That guy from Target" is that a Karen generally has been provoked in some fashion which no one else is can understand and somehow said Karen has determined that 'standing their ground' is demonstratably a sign or act of bravery. This is often perceived by onlookers as some sort of self-empowered policing, whereas "That guy from Target" is more like an unhinged animal, a dog that might become afraid and attack with viscious intent.
"That's that guy from Target - back up slowly and don't make any sudden moves."
"That's the guy from Target and it appears that he is out of pamphlets..."
That guy from Target