I just assumed that demons had means of observing things other than light. Just because we need light to visually perceive things doesn’t mean all beings would need them. Sharks have 6 senses supposedly so their is nothing limiting demons to the five that humans have. Also humans tend to have very poor senses in general and are visual dominated. We can distinguish more colors than most animals, but have a harder time seeing things from a distance, or in the dark. Most of are other senses are joke in comparison. Demons just don’t seem so limited. Many animals can perceive things we can’t, but for reference what is the difference in sensory detail between humans and demons.