I had this issue in Svet, on Stable branch. Could hear zombies, couldn't see any, started to walk around and then starting taking damage. Could hear the sounds of them hitting me, but couldn't see them anywhere, ran inside a building and it sounded like they were following me.

Took dmg inside the building as well until I fired my gun around me just in case, then ran up stairs when the attacking stopped. Seems like they couldn't follow me up stairs. Could still hear them but couldn't see them, thought they might be under the map/ground, but wasn't sure. logged out on top floor of house.

You have to have the bottle you want filled up in your hand, then switch to the other bottle in your hand. Not really a bug, seems pretty realistic to me.

Confirmed, hit by invisible zombie(s) in Svet, ran into a 2 story building, fire my gun on main floor after being hit a third time, then ran upstairs. Once upstairs I could still hear them as if they were at the bottom of the stairs.

Waited for 10 min at the top of the stairs and didn't get hit once, but could still hear them really close. Logged out there at the top of the stairs. Not sure but they couldn't follow me up the stairs