I don't run into this problem too often but its a bit annoying, so basically, its exactly what it sounds like, an invisible zombie, can't see it, can only hear it, It can damage you, and it's pretty near impossible to damage it. I've run into this problems about 3 times so far, died two times, and only on the third time with the help of my buddy, did we manage to trap it in a shipping container for the next poor soul to find. Not sure if anyone else has run into this problem yet but its a bit it took us an annoying 20 minutes and a LOT of rags to get away for sure.