There isn't anything on the platen, it's on this little strip of glass on the left.
When scanning from the feeder, the paper moves and the light stays still, so anything between the light and the paper creates a perfectly straight line in the feed direction. That strip of glass is literally the only thing different between copies off the glass and feeder. If it isn't something on that strip of glass, then it also appears on copies from the glass as well, which your user already tested and said was not the case.
Lay the paper printed side up on the platen, examine the strip of glass immediately left of the line and remove whatever is on it. If it is really stuck, use the edge of a credit/bank card.