I think it definetly COULD be spirits, in some instances.

The thing about outside is any specks of dust in the air or moisture can come across as orb looking too.

Try taking some pics around three am in the morning when you are sitting around inside the house, see if anything pops up. Or hook up a camera to roll during the night. They say three am is "dead time" or when the spirits are most active.

I love this stuff.