Can dogs sense things that humans can't?

Yes I have had similar things happen several times, with all of my dogs I've had at some point or another. They generally don't start freaking out like that, but they will act strange. One time I was outside taking some pictures to finish off a roll of film, and the dogs all started acting like someone they really liked had just walked out the front door, which was closed. They got all excited, and walked around the edges of the porch like they do when we walk out the front door and they "escort" us down the steps, and would look up at something, but nothing was there. It really freaked me out for some reason, so I quickly took the last 3 or 4 pictures on the roll of film of the dogs and porch area, and ran inside through the other door. Whenever I got the pictures developed, there were all sorts of weird pinkish wisps and such in those last few pictures-it still baffles me. So, if your dog is doing that, maybe you should try taking some pictures of whatever he seems to be barking at and see how they turn out. Although it seems like a lot of the time when they bark like that that they must just be barking at something they heard, or thought they heard, so who knows...