That is an interesting problem. Normally we confirm a scanner defect by performing a simple copy and comparing it to a scan to email/folder job. Verticle lines are often are the result of some kind of crud on the glass or the rollers that streaks across the page as its processed. Horizontal lines are typically the result of a bad fuser or another printing component.
I would assume the defect is there on copies but is only noticable when the document is converted to a digital document. Physical outputs must not be able to process and print the defect. You could test scanning to multiple file types to look for inconsistencies and clues. The network wouldnt be involved with this kind of a defect, the defect is happeing on the printer when it converts the scanner data from the tray to a final electronic document. Id bet if you installed this printer locally over a USB cable you would see the same defect in those electronic documents too.
Best bet here is to do a good cleaning. Wipe down all glass, rollers and flaps that may come in contact with the paper as it feeds through the document feeder. If that doesnt do it then consider swapping the scanner assembly.