For the notifications to work properly, you must CLOSE the box after you remove the keys to enter a property. This will trigger a notification to the listing agent that your showing has started.
Do not close the app.
When you are ready to return the keys, tap on the CLOCK icon in the top right corner of the SentriKEY RE app and select END SHOWING. This will send an End of Showing notification to the listing agent and allow you to return the keys to the lockbox.