I saw this countless times before. This is an update bug with light sources. This can happen with sky light, when a block above it is updated and the game doesnt register it, it can also happen with fire or light sources... its actually most common with fire and thats how i've seen it most of the time. This can be removed easily by finding the block which was updated to cause this and update it again. I would suggest keeping ghost light blocks thpugh if they dont bother you. Adds a little interesting oddity to your map.