I am looking for an additional source for geocoding and reverse geocoding. I went to http://www.openstreetmap.org and clicked the button to query my current location. Nominatim returned a "house" with my correct address. When I search for my address, I get the same point. So I figured that someone must have entered a node with building attribute of house and my address. But when I download the OSM data for my state into QGIS, and then create a point layer with it, when I filter the layer on nodes with "building"="house" set, there are only 330 or so in the entire state, all spread out. Like only people who have added their own house have these nodes, perhaps. The majority of nodes that have "addr:streetnumber" set are for businesses, churches, and other POI entries.
Which brings me back to the original question - how does Nominatim know of my house? FYI I also created a layer of the closed polygons, thinking the house may be there, but it is not there, either.