just a thought

Yep, spirits can exist parallel to the physical plane and manifest a likeness more easily perceived.

They aren't going to manifest in the way people think. They can manifest through natural elements, though it is still influence of the physical plane, not actually manifesting physically, even if we see or hear them physically.

Exactly, spirits are not physical nor do they need to be to be perceived. As Eiskorra said they simply affect the physical to allow a perception. They can be viewed however through other means sometimes a bit easier, you can develop the ability to pick up the subtle energies and make sense of them.