I've never seen anything. Even if something existed, I doubt I'd notice. I tend to think that people who say they see stuff are over imaginative, seeking attention, or just plain crazy.
I'm not entirely denying the POSSIBILITY of ghosts, I just have a hard time believing that if they DID exist, they follow people around and hide their cellphones. Wouldn't ghosts have better things to do and better places to go that peek at people through fog or shadows?