I've had this happen twice now since the new patch. I will be playing, geared up, and healthy and then the "you are dead" screen will pop up out of nowhere during gameplay.
The first time I had been playing my character awhile. I logged off, went to bed, and played it again after work the next day. I was in a server with one friend who was nowhere nearby. I played for awhile and fought a few zombies. The only weird thing I noticed is the zombies hitting me a few times but I had no blood or warning message that I was bleeding. I thought this was weird, but I thought "Well, that's one less thing to have to worry about right now." I proceeded to climb to the top inner floor of the firehouse. There I opened up my inventory and was messing around when suddenly "you are dead" popped up on the screen. This was about 10-15 minutes after having logged into the game.
I had a friend login that had also logged out in the same area the night before to go look at my body. On my screen it showed me on the top floor laying there, on his it showed my body had fallen out of the window and onto the roof. It's not possible that I hit V and stepped out of the window because it won't allow that action while I'm in my inventory.
My only theory at the time was that the game was bugged in some way and that I was actually bleeding from those zombie blows. Even though it didn't show up in the lower right hand corner, no physical blood coming out that I could see, and no status message next to my character in the inventory screen saying I was bleeding.
I also, just minutes ago, had it happen again. After my first time I was more cautious. Even after a zombie would fight me and I would either bleed or not bleed I would bandage anyways. Just to be safe.
I had logged in from a few hours of playing this morning and I was, again, on the third floor of a building. A different type of building in a completely different part of the map. I did not spawn in this building, I had made my way into it after a few minutes of gameplay. I was scrounging around looking for things and came across some bullets. I was in my inventory screen about to pick them up when again, out of nowhere, it said that I had died. I had not seen/heard any zombies or players. No shots or blows. Nothing. Just dead. I had no engaged any enemies since logging in. This was maybe 10-15 minutes after logging in just like the first time I had died.