DIAGNOSIS: You have dust or smudges on the underside of the scanner lid (the white backing plate), not the glass. The scanner reads through the glass and reflects off that backing surface.
FIX: Clean the underside of the scanner lid. Open the lid. You'll see a white plastic sheet. Dampen a microfiber cloth with water or mild glass cleaner, wring it out completely, and gently wipe that entire white surface. Let it dry fully before closing the lid.
IF THAT DIDN'T WORK: There could be debris on the scanner carriage strip (a thin, clear strip) along the inside edge of the glass. With the power off, use a dry cotton swab to very lightly wipe it.
HEADS UP: Never spray liquid directly onto the glass or lid; it can seep into the edges and damage the internal mechanism. Always apply cleaner to the cloth first.