(I have the dangerous cities modlet and random wandering hordes mods installed)
I'm having an issue now in the Crack-A-Book Tower T5 POI (skyscraper_02). I've done this POI many times, and I always take the same route. For some reason, when I entered through the window like normal, none of the zombies inside spawned (and I DID start the quest by clicking the exclamation point trigger thing - I got the quest objective in the upper right of the screen too). Then, after trying to make noise and stuff and find them, they suddenly spawned when I was in one of the upper rooms.
I did have some cops spawn outside and enter before the sleepers started spawning. They cops damaged some of the POI areas also before the spawning started. Is it possible that if a block that a sleeper is supposed to spawn on is somehow destroyed before that sleeper spawns on it, that sleeper won't be able to spawn? Some zombies/sleepers spawn in the first area you enter for this POI - they spawn on the roof, on top of glass panes. I think some of those panes were broken before the sleepers actually started spawning. Perhaps that's the issue?
Here's a screenshot after I've cleared the entire POI, gotten the end loot, etc. I've wandered all over this area, and can't get anything new to spawn:
ETA: I failed the quest by moving away from it (after this screenshot was taken), and the yellow marker remained. I had to save/exit, and reload the map to get it to disappear.