Ok, hard to explain how to get to it in the app, but its already built in…
Go to dashboard, click the “Quote bubble” in the upper right corner.
Click the gear in bottom right corner
Choose a mode you want to use to set thermostat to away, ex Goodbye!
Scroll down to Set the Thermostat, choose your nest
set the other options to your liking
Change the mode to, set to away (default)
Set conditions to automatically put in goodbye to your choosing.
Should set Nest to away when mode = goodbye
FYI, I don’t use this, so haven’t tested it, but should work fine.
Also, did you check out the update to the away smartapp, I added a daily away and not away schedule event. So you can remove the nest from away at 4pm M-F if you want and set it to away at 8am M-F and then if any motion is detected in the house during that away time, it will take it out of away.