In the Shattered Sanctum, there's a room with a patrolling goblin, and a drow who is talking to another goblin. Behind the drow, there's a cliff with a war drum on top of it.
I took out the war drum in stealth before the fight so the goblin wouldn't hit it and alert anyone else. However, once I started the fight, the goblin went up and hit it anyways. Nobody got added to the fight, so I figured it just played the animation, and the effect didn't happen. Then, I finished killing those three enemies, and now everyone in the sanctum is mad at me.
Here's a recording of the goblin playing invisible drums:
When I reload my quicksave before the fight, he does it every time, which is how I got the video. I did the fight again and killed the goblin before he could hit the drums just to prove to myself that the other goblins in the sanctum shouldn't be angry at me, and sure enough, they're neutral. (Yellow outline = neutral. Before they had red outlines and attacked before I could even get through the doorway I'm past.)
P.S. Also not sure why attack of opportunity didn't trigger from my cleric in the video... but that's not what this post is about. Maybe she was just barely too far away? But she looks even closer than my fighter is when the attack happens.