We ran about 1,500 cards through the fi-8170 with a thin white line down every single scan before we stopped and actually looked at the feeder instead of the images. Every one of those cards had to be rescanned. The line wasn't on the cards. It was on the glass.
The test that tells you what's wrong
Look at where the streak sits on three or four scans in a row. If it's in the exact same spot every time, left to right, it's the scanner, not the card. A speck of dust, a fingerprint, or a scratch on the glass sits in one fixed position and puts a mark on every page that passes under it, card after card. That's the official explanation from PFU/Ricoh, the company behind the fi-8170: a dirty glass section is the standard cause of vertical streaks on this scanner family.
If the mark moves around from card to card instead of holding still, that's a different problem: dust on that specific card, or glare off a holo or chrome surface, not the feeder.
Holo and chrome cards streak more, on purpose
If most of the streaked scans in a batch turn out to be holos, reverse holos or chrome-style cards, that's not a coincidence. Reflective foil surfaces are more prone to showing lines and glare when they go through an ADF scanner like the fi-8170, according to scanner support documentation covering this exact model. Flipping those cards to scan face-down, and using a dedicated color profile for foil cards in the scanner driver, cuts down on it, on top of whatever the glass and rollers need.
What actually fixes it
Wipe down the glass and the feed rollers. That's the whole fix, and it takes a few minutes once you know where to look. Full steps in how to clean the Ricoh fi-8170. The scanner also has a Vertical Streak Detection setting buried in the driver panel that flags a dirty glass automatically instead of waiting for someone to notice a bad batch. It's off by default.
Official video by Ricoh Image Scanner Global. YouTube loads only after you press play.