I am currently working on a redstone project on a realm, and got very frustrated when creepers spawned in the redstone. Most of the redstone was completely destroyed, and so I looked for dark spots to stop any future spawns. When I couldn't find any, I downloaded a copy of the realm to fly through in spectator, and found no dark spots that would have allowed the creeper to get to that area. I have also had zombies go after the villagers in that area, but there again is no dark areas. The area I am working on is underground, closed off to any outside spawns. While I was assessing the damage, I had more mobs crawl out of areas all light level 7 or higher. I am very frustrated and don't know of any other solution other than to stop playing until this is fixed.