Sony's Xperia X Performance and Xperia XZ flagships both recently caught their updates to Android Nougat, but there were apparently a few more things that Sony needed to do to make the new software into the best experience possible for users, because they have gone ahead and issued a new, much smaller update for both devices. The new updates for each device are extremely lightweight, and don't bring much in the way of a changelog, making it seem as though they are minor smoothness and security fixes, if nothing else. Small tweaks to the build.prop file or blocking off a root exploit or two are common examples of things that find their way into nondescript updates like this.
Most likely, the updates are simply small tweaks for software optimization that their testing teams didn't catch until now, or that their development teams didn't manage to build out in time for the promised timeline of the Nougat updates. In any case, today's updates for the two Sony flagships will bring the firmware version of your device up to version number 39.2.A.0.374. As with any other update, you can check for it manually through your settings menu by going to About Phone, then System Update. If the update doesn't appear for you, a reboot may do the trick. If not, just wait for Sony to roll the update out in your neck of the woods, or download the FTF file from the source link, if you're feeling adventurous and know what to do with the file.
Sony's Xperia X Performance and Xperia XZ, the strongest two phones in their lineup for the moment, were only updated to Nougat about a month ago, with the Xperia X Performance picking up its update on November 29 and the Xperia XZ catching it on the first of December. Given the timeline of the small update here, it could conceivably have contained Google's January security updates for Android, but the small file size and inclusion of the December update make that seem unlikely, as does the fact that the Pixel phones reportedly won't be getting those updates until January 3.