Strange residues after separation of mesh I have modelled this large building (see below). After joining all the elements of the building to one big mesh I separated a part of the building. In this building there are floors and these floors / planes are also separated now. But you see in the encircled area that somehow there are residues ( "borders" of those planes). What are those residues ? It looks like that these residues are the borders of those planes / floors ???



This is the blend file before separation
OK, I think you were in face selection mode, and you have unconnected edges. That is what you are seeing now. Because the edges were not part of a face they will stay unselected when using face select mode. 

PS You also have a LOT of duplicate vertices, 83,559 to be exact, Not related to this, but probably not going to help you.

To delete the unconnected edges, switch to edge select mode, and select one of the loose edges.
Then from the 3D view header Select > Select Similar > Amount of Faces Around an Edge, or just simply ShiftG then A. Now delete all the selected vertices with X > Vertices, or Delete > Vertices.