Invisible Light Source

Light is still appearing at level 15 in a room which is made of solid blocks, has no holes, and no light sources. 

I made a room out of cobblestone, just cobblestone, no slabs or any other blocks make up the room. It has no holes or light sources in it. When I first made the room, it was dark, like it should be, but once I went on the outside of the room and placed torches on the top and around the room, the inside of the room became exposed to a light source. Still no holes anywhere, I went into the room and blocked my entrance hole, and still a very bright light source. Attempted to break the room, and replace all the blocks again, but the "invisible" light source is still present. It is affecting the efficiency of my mob farm, which is the purpose of the room.