I got disconnected from the server (maybe a server restart) and after rejoining multiple invisible zombies attacked me and dealt damage. I heard their voice effects, footsteps etc.
- Legacy ID
I noticed that I heard random zombie sounds while I did a run through the woods till I died of hunger north of Kamenka. Someone I play with noticed that this started for him once his hunger and thirst messages starting getting urgent. Could be a related issue.
I had the same issue with build 35.
Me and my friend spawned at the same time in two locations. Me in Gorka and he near Polana. I started heading from Gorka to Polana following the road. I assume that some zombies spotted me near Gorka and followed me to Polana. I was able to see the zombies all this time, but my friend who was waiting for me there in Polana was not able to see any zombie even when the zombies were running for him. Finally I killed them. I hope maybe this helps.
Another note: It is possible that my fried had some desync before arriving zombies to Polana, but at the moment the zombies were near him he had no connection problems and still not able to see them.
I think what happened OP is that you were lagging hardcore, hence why you were disconnected in the first place. I think this because I had this happening, and tried unplugging my roommates connection(he was downloading), and the zombies instantly appeared. Weird that the zombies need so much packets(?)/bandwidth though.