Casper the A**hole ghost stole my Grump

I was in a normal game with skarner clearing grump and my bot lane left to go to lane after leashing and a few seconds later when I killed it the weirdest thing ever happens, I get half XP then the green trail that rewards the killer of a large monster with health and the gold flies off towards the single bush at the tier 2 turret and no one was there. I've already concluded it wasn't a player because there were no invisible enemies and the enemy Ekko was at his Red so it wasn't him so the only thing that I can think of is that either Rito or Casper decided to be a massive D*** and steal my grump