Merriam-Webster details the word Sentry to mean a guard or watch, specifically a solider standing guard. So, the straight-forward interpretation of Sentry Mode would be security programming that will keep an eye on the car, using the cars several cameras.
Musk first said that the new feature would come to all cars with Enhanced Autopilot, but that could be interpreted a few ways, considering Enhanced Autopilot is an add-on option that costs thousands of dollars. Musk cleared it up to a different Twitter user, saying that Sentry Mode will come to all cars with AP2+ hardware, as it is a security feature that should be widely available.
Tesla Sentry Mode coming soon for all cars with Enhanced Autopilot https://t.co/x2buQWiABX— Elon Musk (@elonmusk) January 22, 2019
Since it's part of safety & security, all cars with AP2+ hardware will get it— Elon Musk (@elonmusk) January 22, 2019
The idea of using the car's cameras that were first built-in for driver safety nannies is not a new concept. Sentry Mode could even be considered an extension of the dash-camera capabilities that Tesla added in 2018. These updates have been introduced via over-the-air updates through Teslas infotainment systems.
Furthermore, Musk responded to a question about when another feature Advanced Summon would be available:
As is usual with Musk's random declarations, there is no certain timeline for when this could happen, but it sounds like it will be added at no extra cost. Stay tuned.
Almost ready to roll out. Regulators just approved.— Elon Musk (@elonmusk) January 22, 2019