Probable ESP - I was in a barracks building in Zeleno (I spawned in inside the building after a server restart). Door was closed, and I had a tripwire on the door as I was eating IRL just chilling.

So I hadn't been outside at all since the server restart, or for a while before that.

I hear sounds of people outside - I think they were fighting zombs, bandaging and eating. Then they throw a grenade through a window and kill me.

But there's no way they could have known I was in there - it was a first-person server and I had been in that building for AGES.