Yes dogs can see spirits. And can hear things much louder than what we do. For example, My dog shits itself when he hears fireworks.

I do think so yeah. I mean, every animal is different, and has different sense's of hearing/seeing. So they could see things we can't, or hear things at different frequency's than we can.

And if a dog is sitting there, looking at nothing, barking and staring, surely there is something there, right?

So you are basing the fact that dogs may have a sixth sense due to horror movies?

As stated before, sometimes when a dog goes outside, the hyped smelling, seeing, and hearing senses can sense an animal from far away that you may not be able to see or hear. They will bark to show dominance or whatever specific reason they feel necessary.