Zombies should not be able to spawn within a line of sight to the player (no matter which direction the player is looking) they could spawn in a room (or behind a blocker) behind you though and run out of said room/blocker to attack you.
Its possible with a high ping on a server you could see a zombie spawn as the your perceived position is different to what the game knows as you last position.
Either this or some custom maps such as metro tunnels impliment a custom spawn (I believe) for zombies to spawn in a specific place (whether the player can see it or not) this would not be the case for official maps however.
Hoobalugalar_X
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