While I was playing for about 2 hours or more, I heard as if there were zombies chasing after me from time to time and it was quite annoying because there were none behind me.

Also, while I was looting military base near the north airfield, I heard gunshots but as I checked around and spectated the area for a while, there was no one there. I'll leave the possibilities to what caused the gunshots open, might be a player who logged out.