Hallucinations are where someone sees, hears, smells, tastes or feels things that don't exist outside their mind
They're common in people with schizophrenia, and are usually experienced as hearing voices
Hallucinations can be frightening, but there's usually an identifiable cause. For example, they can occur as a result of taking illegal drugs or alcohol, a mental illness, such as schizophrenia a progressive neurological condition, such as Alzheimer's disease or Parkinson's disease, loss of vision caused by a condition such as macular degeneration – this is known as Charles Bonnet syndrome
I can't help feel a little sad that anyone having, or believing, anything outside the norm is said to have a physical or mental illness.
I do not particularly believe in the supernatural and certainly have never had any experience personally. However I live in an old house built on an ancient graveyard. Over the years we must have had a dozen or more people say they have felt or seen the presence of an elderly man and young child when in our garden but only in the rain(???). This is people of all ages that do not know each other, or people such as tradesman that we do not know at all, describing a similar vision. As I said I have never seen or felt anything myself. However, reading this thread it seems just as unlikely that ALL of these people suffer from a condition or mental illness or take drugs, than it is that there might be something in it.