I have had these shadow people experiences before too. Usually very thin. My husband saw them too in the house. He never had any paranormal experiences until we met. They were really creepy to him too. When he first moved in, sometimes I would come home to every light in the house on, because he was spooked. The oddest thing is when one happens to walk in front of our big tv while it is on and they are very opaque. I haven't seen any in over a year, but they are deffinately interesting. Some people associate them with old hag syndrome or sleep paralysis, but mine never seemed to be related. I have experienced sleep paralysis before, but only in the most scientific, non-paranormal way. I knew what was going on and didn't have paranoia or feel a presence. This is when some people say they usually have the shadow people experience though.
You said you got a good look for a couple of seconds. A lot of times people see them from the corners of their eye. Almost like they are trying to hide and just observe without being caught. I am not really sure what they are all about, but there are a lot of interesting theories. I have even heard of a specific kind that supposedly always wears a hat. It is also supposedly taller than the normal ones. Weird huh!.. I haven't seen one of those.