Rear camera does not record dash cam.
Also, you are combining two different things (sentry mode, and dash cam) into one function, when they are separate. They both record to the same place, but are different features.
Dash cam = camera that records when you drive. Always recording and writes over itself after 1 hour of driving. If you want to save something from the dash cam, you push the little download icon and it will save that footage instead of writing over it.
Sentry mode = mode which can record while the car is parked for security purposes. It can be setup to auto activate or not. It always saves and does not write over itself. The "sentry detected" you see if you see one alert when you get back to the car is likely YOU as you walk up to the car.
In neither mode does the rear camera record (yet). To get more information, you can search this site for "dash cam" and "sentry mode" as two separate searches, and read the threads on each.