My SD card very quickly became corrupted when I enabled Sentry mode. I do not believe it overwrites or clears space the same way the dash cam does. That being said I only had a couple gigs available on the drive. Sentry Mode seems to need way more space than the dash cam. I think it needs a sensitivity feature in future because I turned it on in my garage and it recorded a lot of useless video. I think once it gets dark the cameras seemed to detect the slightest changes in light as motion and start recording.
Sentry Mode videos are written to a "saved" folder and not automatically overwritten like Dashcam videos.
With the advent of Sentry Mode, three cameras are now being used for all (Dashcam and Sentry) video recordings, so storage space definitely gets eaten up faster than when only the front camera was used.
As for the extraneous videos, I think that's one reason Sentry Mode always reverts to OFF after every session. It's too easy to forget to turn it off and you end up with a lot of junk. Better to manually enable it just for the occasions/situations where it's needed.