You can do what you want if you use my Nest device type (Nest device type with a bunch of new features) with Pollster ([OBSOLETE] "Pollster" - A SmartThings Polling Daemon). My updated Nest device type exposes Nest presence as a SmartThings presence sensor and Pollster refreshes the Nest device on a schedule. This allows you to create a hello home action that will automatically run when everyone leaves, where everyone is your Nest presence sensor.