Usually that type of thin black vertical line is caused by a bad Display. Unfortunately some display issues can be intermittent. You can try moving the display back and forth to see if the line appears with the movement of the display in case the LCD panel has a loose internal connection.
If the line comes back connect an external display and enable mirroring. If the line only appears on the internal display and not simultaneously on the external display, then you have confirmed the built-in Display Assembly is bad.
I would be concerned too if you only have four months left on the warranty. Just make sure to keep the documentation that you had Apple or an AASP examine the laptop for this issue before the warranty expired (as well as the pictures) so it is a known pre-existing condition. Even so you may find it hard to get Apple to repair it for free once the warranty expires even though you took the laptop to Apple or an AASP during the warranty period for the issue. I'm sure the line will come back at some point, but how soon is the question.