Does it also occur when copying?
Does it occur when using the printer's control panel or when scanning using the computer?
It is occurs in all the above are you scanning the document using the ADF or laying it on the flat platen glass?
If using the ADF, lift the cover and try cleaning the glass and the 'strip' at the end of the glass below the ADF. It only takes a speck of dirt, grime etc to create the line in the scan.
Do not use household cleaners etc to clean the area. Use a lint free cloth lightly moistened with plain water. You don't want any excess liquid entering the printer.
When satisfied, scan or copy a plain sheet of paper through to see if the spot has been removed.
If using the platen glass ensure that the glass surface is also clean from marks etc.
Hopefully this is of some help.