Lookout scans apps installed from Google Play by default. The File Security feature scans files on your device as well.
In the Lookout app settings you may have seen an option called "File Security," which is on by default. (Menu > Settings > File security) This feature scans all files you install—such as files placed directly on the SD card or sideloaded. ("Sideload"—as opposed to "upload" or "download"—refers to the process of transferring data between two local devices, usually between a computer and a mobile device.)
This feature is great for people who have changed their Android settings to allow non-marketplace apps on their phones, as it provides an opportunity for threat detection prior to the app or file's installation. It also allows Lookout to actively monitor file systems for changes and to scan SD-card loading or sideloading events for possible threats.