These are faint, somewhat wandering, hairline seams which if present (usually) are sporadically visible on the sides of machine-made bottles.
The ghost seams are caused by the parison mold parts and if visible enough will be "attached" to the vertical seams in the finish.
Click on the picture to the left to view an illustration which shows both of these seams or click machine-made finish to view an image which shows well the seam below the finish.
Both seams are quite diagnostic of machine manufacture and are usually visible, though the seam at the top of the finish can be hard to see on some bottles - especially if the finish was fire polished.
Lot of good folks here with alot of bottle knowledge.