Google will automatically delete all of a user's Android backup files — stored in his Google Drive account — if the user does not use his phone for two weeks.
After Google detects this period of inactivity, it will start a 60-day counter for old Android backup files. After that counter reaches zero, Google will delete the backup files from the user's Drive account.
The auto-delete function was discovered this week by a Reddit user who used it to create backups for a defective Nexus 6P. The user sent back the phone, and while he waited for a replacement, he saw that his Nexus 6P backup files stored were marked for deletion.
A Google Help page offered very little information on this mechanism. Making matters worse, Google does not show any warning about the auto-deletion mechanism when users save a new backup. Google also doesn't send an email or other type of notification when the backup files are marked for deletion.
Google backups include very sensitive data such as Apps, Call History, Device Settings, Contacts, Calendar, SMS (Pixel phones only), Photos & videos (Pixel phones only).
People who rely on Android's built-in Drive-based backup system should keep an eye out on the Backups folder. Storing backups offline or using specialized backup & restore Android apps is an alternative.