The Rice University Neologisms Database

Apparent meaning: an unknown person; particularly an unknown person whose presence cannot be explained. This word comes directly from the adjective random meaning ‘purposeless.’ Possible Reason Used: The term is convenient, readily understood, and is uncomplicated. It seems more casual than saying “a bunch of people I didn’t know” and it also connotes that the people in question not only were unknown to you but their presence was inexplicable. So that the term random achieves a convenient double implication, the person in question is not only unfamiliar but out of place.