There is an unused human zombie enemy in the game files for all three games. It is thought they were meant to be the end result of zombified stalkers who have lost the ability to operate mechanical devices.
The PDA Encyclopedia also contains an unused entry for them;
"The Brain Scorcher strips its victim of mind and reason, leaving only an empty, mindless shell."

While not included in the games, all the necessary files are contained in the installed data, making them easy to mod back in.