The sense referred to as your 6th sense is usually the ability to feel the presence of objects and moving things in the environment. It can take place due to a sense of heat or sound but recently scientists have proven that the brain produces enough electrical activity and considering it is highly sensitive it can possibly detect the presence of other objects nearby. For example, when you are near someone and they start to laugh it can make you laugh as well - not necessarily because it's funny but because our brains can communicate enough to induce the other person to laugh . It has not been proven to work 100% of the time and there is not much evidence to back it up even now so it is not regarded as an official sense. I do however believe it exists since I can feel things coming somethings but I would not take that too far personally for my own safety - seeing ghosts is ridiculous nonsense! No one has ever managed to provide concrete evidence to show that ghosts exist.