Either is fine. What makes sense to me is kind of a combination of the two. It is, after all, the behavior I'd expect from the sleepers, instead of them staying dormant even when I shoot the wall next to them but getting up as soon as I cross an invisible line.
For me, sleepers aren't necessary at all for a good stealth experience, though I think their existence is kind of cool and atmospheric. I'd be happy with a building full of zombies wandering around and I'd have to figure out their behavior or find a good hiding spot, or snipe them, to avoid notice. Maybe I peek in through a hole in the ceiling and pick a few off that way, get a few more through some open windows, etc.