Zombies are able to detect, and aggressively react to players, as well as wild-life, at any distance regardless of the Player's stance(prone, crouched, standing). At times, they are able to see through buildings and obstacles as well.
Issue also manifests itself as having zombies simply appear out of the blue, running towards the player. Unclear whether they detected the player through a wall or were spawned already aggroed to the player.
Most problematic as of the recent update to Stable, 0.47.124641. {F30768}