Occasionally i will imagine something for example, words on a wall or an object that is not there. It looks completely real. I then focus and double check and it completely vanishes like it wasn't there. I thought it could be due to lack of sleep and tiredness but i also experience it when i have had normal amounts of sleep. For example, today i saw a girl carrying a large WHITE 'art' type folder down the street, when i looked properly the girl was still there but she didn't have a folder s