This is working as designed. The Dell ExpressSign-In 1.0 (via Proximity Sensor) only functions when the Latitude 7320 Detachable is connected to the keyboard.
I noticed that the Proximity Sensor only works on Latitude 7320 2-in-1 with the keyboard attached. When I put the tablet 'part' on a stand, the whole 'presence awareness' thing does not seem to work. I wonder: is this by design? What if I put the tablet 'part' on a table and walk away - shouldn't such event also make the Proximity Sensor lock the laptop?
Latitude 7320 Detachable, proximity sensor, tablet mode?