Answer for Home automation with Google Home and Z-Wave from the ground up

Oh, that’s a great question.  Ya, it looks like the action of locking or unlocking is not showing up as a possible trigger of a routine.
 
I wonder if there’s a SmartApp somewhere that would give you that kind of functionality.
 
However, I would probably recommend you not go that route, because if someone breaks into your house buy guessing the passkey or by watching someone enter in the passkey, then you would want the alarm to go off in that case.
 
What you should do is get some SmartThings Arrival Sensors and give them to your cleaning lady and nanny to put on their keychains (or keep in their purse).  You can add those in Add a Thing > Sensors > Presence Sensors > SmartThings Arrival Sensor