Even if they dont want us to know, just make the zombies awake and moving around.Except, maybe they don't want you to know in advance. Asking for some sort of notification letting you know what happened seems reasonable. Asking for some sort of warning letting you know whats going to happen is probably not.
Im not sure if theres anything in this game what looks worse then some random shambler doing a full 180 because you entered the room in full stealth gear. Also it doesnt need to be actual warning, these things can be used to set an atmosphere and also its quite possible to suprise a player without going for ceiling zombies and stuck in a furniture setups.
Examples:
- Graveyard and other places where its ambient to find dead in the ground could have some random dead hidden by an instabreak dirt block so the player falls in the hole and has to wrestle with the undead there. Quite scary deal.
- If every dead could crawl we could have some fake ventillation point like looking places all around factories and bunkers where the dead crawls out and rushes at us in feral mode after we passed throught that point a bit. Makes people paranoid towards walls.
- One dead right behind where a door opens up so hes in coverage is always a good scare.
- The runner strike i mentioned has no signs expect the runner rushing at you and exploding alerting every dead in the building.
- Turn all the fake bodies ingame into zombie looking ones and randomly make one of them into zombies when hit.