Terminal command to determine the last time a MacBook Pro was unplugged from AC Power?
You might be able to use ioreg to determine the cycle count of the battery. A low cycle count indicates that the battery has not been discharged and charged very much. 

ioreg -l | grep Capacity


"Cycle Count" shows how many times the battery has been cycled. You can read more about battery cycle counts here.

Another idea is to use pmset to log the state of the system's power. You could, in theory, have that dump to a file to investigate after some time has passed.

Take a look at the man page for pmset. 

pmset -g pslog will log the state of system's power. As the device is unplugged or plugged you'll see the log update. This could show how frequently a notebook is unplugged and charged. If it remains plugged in for long spans of time, or is often drawing from 'AC Power' then you'd know if it's mobile or not.