Per the MODEL 3 OWNER'S MANUAL:
The TeslaCam folder contains three sub-folders:
• Recent Clips - The footage in Recent Clips continuously loops in 60-minute cycles whenever the cameras are activated. Therefore, footage is overwritten every hour unless you save it. When an event is recorded, one video is recorded for each of the front, rear, left, and right cameras.
• Saved Clips - Contains all recordings that you have manually saved using Dashcam.
• Sentry Clips - Contains the last 10 minutes of footage from all Sentry Mode events that have triggered an Alert or Alarm state. The footage from each event is labelled with a unique timestamp.
NOTE: As the USB flash drive runs out of available space, the oldest footage in Sentry Clips is deleted to make room for new footage. Once deleted, you are unable to retrieve them. When the flash drive is full, Sentry Mode and Dashcam can no longer save video footage. To prevent the flash drive from getting full, you must regularly move saved videos to another device, and delete them from the flash drive.
https://www.tesla.com/sites/default/files/model_3_owners_manual_north_america_en.pdf