Today, I opened up an application (coconut battery) to see check how my battery is going along, health, etc. It said that my Mac had an unknown manufacture date and no serial. I opened up About This Mac, and saw that for some reason, it says my serial number is "Unavailable", and it doesn't show the exact model of my mac (It just shows "MacBook Pro" instead of "MacBook Pro, Retina Late 2013" like it used to, even when I click on it) I do have the serial number written down, and checked my warranty with Apple. I do have a valid purchase date, and Apple Care for 1 more year. I have had the display replaced under warranty a few months ago, but I am pretty sure that that did not do it.