I export svg files from Sketchup (using flightsofideas plugin). The lines are exported as paths. I noticed in inkscape that there is a hidden object behind the visible. For example, I exported a circle, then in inkscape clicked on it to select it then used the arrow keys to move it over. Then if I use the mouse and select a wide area, it not only selects the visible circle, but some hidden one in the original position. I don't know what this hidden one is, but I would like to find an easy way to delete it. What is this object? There may be many of these hidden objects in my exported files, so I'd like to delete all of them. The reason is I use this svg with my laser cutter and these hidden objects cause some problems. Here is a screenshot of the hidden and visible circles selected.


I also attached my svg file with this circle in it.

Those seems no hidden objects but paths with only one noded straight segments.
You can use the xml editor (Shift+Ctrl+X) to get rid of them one by one.
Or select all objects and deselect the paths you want to keep and delete them.
Would be impressed if that export feature could make curved paths from the face's straight edges.
Or to handle hidden parts and z-buffer use right.

Another idea to try with 3D to svg: use the 3D polyhedron extension.
You export the 3D to .obj and put it under inkscape/share/extensions/poly3Dobjects folder and you can render them in any ortographic view set with the extension.
I'm using that recently as part of my working method.
Needs some working with adding curves to the segments though, but it's quite effective.
One example:
http://inkscapecommunity.com/ic_gallery/albums/userpics/10014/rb152.svg

Any way to tell inkscape to hide all my visible objects, then I can just group select all these nodes and delete them.

It does worth a try to export to obj and use the 3D polyhedon extension.
You could avoid all the trouble of making any unwanted objects.