If your printer has an ADF glass bar, it might be a strip of glass to the left of the flatbed / main glass plate on which you scan documents / images.
Example 1
NOTE: Image excerpted from Improving Scan Quality
Example 2
It is possible that the "spec" is damage of some kind, in which case it cannot be removed.
It is possible that the "spec" is actually something else, that is, dirt under the glass or located inside the printer.
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