Its kind of hard to prove that, but mob are spawning into iluminated areas. This places are dungeons and even the lab below my house. I took some screenshots because its getting worse and I've been surprised several times in the last days by skeletons and creepers. I thought that would be safe places because I learned to put torches to repel monsters.
The first screenshot shows the place where the zumbi came from. In the second one Im on that place checking the lights. The third shows the same place one block below to check the light on the foot.
I noticed that walking by this area the framerate drops and its kind of laggy.