Take a look at the Aqara hub and motion sensor. You can create automations on thier hub that can send a push notification if their motion sensor is has not sensed motion for a certain amount of time.
I have not used this feature myself, so I don't know how well it works, but the functionality is there.
r/Aqara is the subreddit for Aqara. You can buy a hub and a motion sensor on Amazon.
If you cut power to your Hue bulbs, using the Lutron switch, you will not be able to manipulate them in the the Hue app, or Homekit, anymore.