Lurker zombies only spawn inside houses, shops, and other indoor/underground areas. If the spawning system is capable of it, they should prefer corners and rooms without windows.
Lurkers do not respond to sound - at all. They are effectively Deaf. Lurkers only respond to sight. Lurkers move faster than regular zombies once they spot the player, and their speed is not affected by rough terrain. They do not bite but instead have long sharp claws that they cut the player with. In melee combat they are moderately quicker and more dangerous than regular zombies but have similar health.
Lurkers were the inhabitants of whatever building they are lurking in, and are dressed accordingly. Lurkers could be found in housecoats in houses, or riot gear in police stations.
Lurkers avoid sunlight and will not exit a building into sunlight during the day. At night, however, they will chase the player doggedly.
So the main idea here is to have a kind of zombie which remains inside buildings. While other zombies will rush out and smash windows at the first sign of trouble, lurkers will skulk around inside, waiting for the player to enter before attacking. This helps keep indoor areas mysterious and dangerous, and prevents whole areas from clearing out after one gunshot or a cleverly thrown firecracker. After a big fight in the street you shouldn’t feel totally confident about stepping into that nearby house. It should still feel risky. The purpose of the Lurker is to add to the risk of exploration and looting.
