A friend or confident of your ex that goes to the same parties/gatherings as you and informs your ex what you were doing and who you were with at said function. Ultimately, your ex will show as to send and icy cold/frosty stare to you that would make kittens cry.
"Dude don't look now but Tina just got here."
"Seriously. Dude there's a reason why I go to the university crew team parties now, she doesn't know anyone here."
"Don't be so sure dude, Valeria is here. She's sleeping with some dude on crew."
"Damn ninja popping out of the planter over there must have talked to her."