I believe in them. But I dont believe everyone has the ability to see them. Me being one.
However there have been many occasions whilst walking or driving when ive done a sudden double-take, convinced I saw something or someone. When a second glance reveals nothing.
I think animals are more tuned in to things like this. For instance my house is quite old and has been occupied by many elderly people during its time, some of who died here. My dog, who only stays with me weekends will sometimes act strangely and refuse to go into certain rooms. He normally sleeps in my bedroom but on some occasions gets restless and goes back downstairs, or refuses to come upstairs at all.
I of course sense and feel nothing.
So perhaps it is nothing
lol