Do you know what software is was created in?
I know I have had this issue in the past and I just ignored it, right wrong or indifferent.
I chaulked it up to SW misinterpreting the model upon importing it. Because there is
no way to fix it.
righ mouse click on the import & perform import dianostics
..it is most likely a open seam which falls within tolerance... but may bite you later!?
other than using the wizard to try and auto mend the seam(s)... one trick is to export the body in another non native format like IGES, STEP or SAT... then, re-import... depending on the tolerance on the boundary... it may close it up (or make it worse) during import?
The easist solution would be, iges out & back in, the iges translator performs some automatic reparis for you