I use @RBoy’s dth and smartapp for exactly that purpose. They work great!
I’m using the 2nd gen Iris keypad.
When a visitor uses our keypad, LUM locks/unlocks the keypad (it’s seen as a lock too) and then I use that lock in an automation:
When the guest leaves and uses the keypad, it locks it and arms STHM.
If you don’t want to use a keypad with the dth and smartapp, you could also give that person a presence fob attached to a key chain. We do that too:
When the presence sensor leaves, and all other presence sensors are gone (like my wife and I) STHM arms.