Every single month Google releases a new software update to its Nexus devices and to the Android Open Source Project which patches security issues and bugs that can affect the security and privacy of Android, and back in the beginning of the month Google released the latest security patch for their lineup of Nexus devices. April's 2016 Nexus security update changelog lists the details of what this patch focuses on in terms of the issues, and the factory images for the security patch were pushed out the very same day for those who were inclined to flash the software to their device instead of waiting for it to show up on its own.
The software push for these security updates are usually a bit of a slow rollout and take more than a week to start reaching the wider range of users with compatible Nexus devices, and as of today it looks like the OTA files for the April 2016 security update have started to hit the web for users to download if they haven't already received it over the air officially or flashed the factory images. The OTA files have been posted up to GitHub and can be downloaded from the repository there, although it is worth noting that at the current time not all of the compatible Nexus devices which are slated for this month's security update have OTA download links just yet.
As of right now users with the Nexus 6P, Nexus 5X, Nexus 6, Nexus Player, Nexus 9 LTE model, Nexus 9 WiFi model, Nexus 5, Nexus 7 2013 mobile model, and the Nexus 10 can all download the correct OTA file for the April 2016 security software update. That still leaves those with the Nexus 7 2013 WiFi only model and users with the Pixel C tablet waiting on a link for the OTA meant for their device, although if users really want to flash the software now instead of waiting for the OTA or the OTA download link, they can still flash the factory images for both of those devices. Users should of course only flash these OTA files if they're aware of how to complete the process.