So if I pass by a motion sensor on my way out of the house and arm the alarm as I leave, I am likely to see that motion sensor as Activated for some amount of time after I have armed the alarm. Is that true?
This is correct.
See this thread for details on how it works:
This at a glance status you are referencing in the app and website is for sensor activity monitoring. It does not affect how the motion detector functions with regard to reporting alarms.
This delay is in place because with an at a glance status, the actual current status of a motion detector is meaningless.
Doors/windows open until closed. Motion detectors report activity momentarily. They do not remain open until activity stops, or report closure.
The true current status of a motion detector would only show as activated the very moment it is first tripped. Checking it in ADC it would always be idle.
If your motion detector activated delay was 20 seconds, that would mean the only time you would see that motion had occurred was if you checked the app within 20 seconds of it occurring. Such a timing would only really have application in testing the detector, not everyday normal use.
Instead, that activated status remains, allowing users to confirm that activity has occurred on that detector within the past 30 minutes if it shows as activated.
So in other words, the activated/idle activity monitoring status of the motion detector is intended for positive confirmation of activity having recently occurred, not confirmation of a current lack of activity.
This does not impact alarms or how the motion detector communicates to the panel. The sensor may still have an activated status in Alarm.com but if you set off the alarm at the panel by that sensor, your alarm is reported in ADC immediately.