Yesterday I returned home, unlocked the door, disabled the alarm, checked that the current mode showed “Disabled,” and opened the door. The alarm sounded! My wife entered the PIN about the time I got the very prompt and professional SMS message from Noonlight and shut down police dispatch.
I mentioned the incident to Support in another context but did NOT report it since I can live with the behavior.
My guess is that either (1) it takes a few seconds after Disarmed shows as the current mode for the system to actually enter Disarmed mode; or (2) the system remembers that the door was unlocked when the system was in Away mode; transitioning to Disarmed mode does NOT disable the alarm that occurs when (1) the system is transitioned to Disabled mode and (2) the formerly locked door is opened. [Originally I wrote “unlicked.” I was tempted to leave that, since both words are indeed accurate. 
]
My lesson learned:
- Disable the alarm.
- Pause a few seconds.
- Unlock the door.
- Open the door and enter.
In particular, do not omit #2. And, do not reverse the sequence of #3 and #1.
Actually I’m using an Enter Location rule to unlock the door and cannot ensure that #1 precedes #3. I may have to reconsider use of the Enter Location rule.
HTH!
Cheers,