In Ranked (and maybe Quick Match), right when the match is over and your team wins, you see the player menu and right away 1-2 names drop out.
They basically LOST XP points (-2000 for you!) and couldn't even wait for 20 more seconds.
What doesn't make sense is, you see a post-post game menu anyways when it searches for a new match, and it shows exactly who quit at the end (name is highlighted) and how many kills/deaths they have.
Needless to say, most of the quitters did horribly and must be so embarrassed