Now, this just happened to me. What happened is that the server was lagging a lot, so I decided to join another server. I was inside a third floor of an apartment building at the time.

When I joined another server, I spawned where I previously was, saw that there wasn't any one around (Didn't hear any footsteps nor moans). I walk in a room and I notice that I have the red broken chain on my UI. Ignoring it, I proceed to pick up a map that was on the table. I saw that it was lagging because my character didn't pick it up. Then suddenly, YOU ARE DEAD.

No gunshots, zombies can't go upstairs, I have no freaking clue what happened. I believe something badly happened in the game that it killed me instantly.