I have yet to see a slime too, after ~30 hours of play.
For why you are getting spawns, I have three theories: - there is a tunnel you haven't noticed through which the enemies are coming. (it could be above you, and so they fall into the room) - or, while you think you have well-lit the area, the light is actually not quite reaching the center of some large area, and it is dark enough there for enemies to spawn - or, there is a spawner very close to you, on the other side of a wall, and it is spawning enemies into your room. I'm not sure if this is possible if your room is well-lit though.
--- When YHWH destroys people, it is nobody's "fault," but for Him to do so, is right, and that is why He did it. - My sister
Ya i was playing peaceful for a bit and a bunch of slimes did spawn in my mines on the bottom of the world. So i have no clue how they spawn. --- PSN: Johnsis Won't change until Braves win World Series (22-04-08)