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