[GAMEPLAY/CLIENT] Enemy Sion ult sound heard from FOW

Last night I played against a Sion on the enemy team. Whenever the enemy Sion ulted, I could hear the sound that plays when he begins charging with his ult, regardless of the fact that he was not visible on the mini-map nor was he nearby.

I'm not certain whether this is a gameplay or a client bug. This was the only match in recent memory where there was an enemy Sion on Summoner's Rift. I was also suffering from lag during this game.