Second option. They still there, just the model disappears.
Edit:
I've mapped the whole map brushwork and so, then I started to put entities, on the start, when I had just a couple of few entities, the monsters wasn't disappearing, I dunno what I did wrong for they start to not be rendered, I'm not using any big sprite or anything.
Here's an screenie of the whole thing on 3d Hammer Camera, if you find it usefull.