when you learn how the human eye and brain work you can easily see where people might see things that are actually not there.
There are usually three explainations for these things.
1) the mind playing tricks on you (assembling data incorrectly)
....a) the human brain likes to see patterns and familiar things
....b) evidence that it is a trick of the mind or psychosis. No one else in the room ever sees them.
2) They are really deamons that once you know of them you can see them regularly. Sort of like when you buy a new car and suddenly you see them all over the place. Once you see a deamon you see it all over the place.
3) Psychosis, You are flat out nuts and you are seeing things because your brain is broken. Everything you see, hear, feel and smell are all evaluated and processed by your brain. Your brain if broken can make anything seem real or make nothing into something. You are your brain.