In Windows Update, when you haven't installed patches for a little while, you will receive a notification in the system tray and also in Windows Update Settings that says "Your device is missing important security and quality fixes." In some cases, your computer might say this message, but when you check for updates from Microsoft servers, it says there are none available.
The possible solution is that your computer is only checking for Windows updates and not for updates to programs such as Microsoft Office. Even if you are up-to-date on updates for Windows itself, the message will still appear if you are behind on Office updates, etc.
To change the setting, go to Start > Settings > Update & Security and click the Advanced options link. Make sure "Give me updates for other Microsoft products when I update Windows" is turned On. After you do that, go back and recheck for updates. Once installed, the warning message should be gone.