I've been thinking about the fact that spirit (and UFOs) so often
show up on "film", but weren't seen at the time.

With this footage, it could have a lot to do with light and the limited way the eyes see, particularly in low light (rods vs. cones). A camera can capture more than the eye and then expose more using contrasts, infrared, and through use of algorithmic processes, such as local equalization.

Photographs that capture unseen entities could be due to the entity's vibrational rate and the waves it produces. These waves may be more easily captured in a still moment, like with the many UFO photos in which people did not see anything in the sky while taking the photo. Whereas the eye may tend to override the vision, when viewed in live motion, with the mind "filling in the blanks" as it does and simply process it out. Lots of possibilities.

When I watch Most Haunted (one of my favourites, though there are so few ghost shows available here) I record it and sometimes watch bits of it frame-by-frame. So much more is captured than what is visible even with the moving film. The darkness itself when filmed is constantly shifting and changing as if it is alive or full of life. Which I believe it is.