For me orbs themselves are nothing to get excited about, I believe they are, as some say, the first stage of a spirit manisfestation - however if you look at their behaviour, then you might be getting somewhere.
If you consider how dust behaves when it close to a TV screen or anything with a static charge and the look at orbs it can kick up a few ideas.
Recently watching an investigation a girl was on video being "followed" by an orb ... this may simply have been dust (or similar) attracted to a charge on her cheap polyester top
So if a spirit walked through an empty room (for example) whatever the make up of the spirit is (electro-magnetic etc?) could this affect the behaviour of particles in the air?
I've taken a picture in a place I felt was very active, one second not much going on .... 4 seconds later, hundreds - taken from the same spot.
So my opinion is that orbs are nothing more than dust etc but their behaviour could be a good heads up to activity.