Proximity or geo fencing, Arm and no Disarm?

Trying to talk my wife into letting me use this to replace our old ADT alarm box. I use a Smartthings hub. Is there a way to have the alarm activate when all three family mamber are out of the geo fence known to smartthings? If so is there a way to have it set when we leave but do not disable alarm, having us use the keypad to disarm?

I got the ok from my wife to drop ADT and I picked this same concept as well but after she had already agreed. I think it can be done based on what I have researched so far but haven’t been able to test since all my components are in the mail. Since you have the hub you should be able to set your phones as proximity/presence sensors to trigger a notification. To arm the system the trigger action would be to a Action tile sequence instead of a notification.

Here are some links. Just eliminate the steps for Home mode/I’m Back/Disarm.

The best way to use your new Smart Home Monitor system is to automatically arm and disarm the system as you come and go based on presence. To set this up, configure Routines to set Smart Home Monitor to Armed (Away) when the Goodbye routine is triggered and Disarmed when the I’m Back routine is triggered. You can then automate the triggering of these routines with Everyone Leaves and when Someone Arrives.

What you can do with Routines
There are four default Routines:
• I’m Back! : You have returned home (triggers Home Mode)
• Good Night! : You are going to sleep (triggers Night Mode)
• Goodbye! : You are leaving the house (triggers Away Mode)

Note: Routines can be used to Arm and Disarm Smart Home Monitor,

The issue is that SmartThings presence sensor is highly unreliable.  Use it for a week and you'll see that it often reports the wrong status and doesn't pick up on the fact that someone arrived or left.  

Is your experience based on using the phone as the sensor or the separate ST sensor or both?

My experience was with using the phone as a proximity sensor within SmartThings. Pretty useless. If you look on the SmartThings website others confirm as well. I think if you want to do it your best bet is to create your own rules based on multiple sensors and even time of day etc. one option that I’m considering is using a new smart garage door sensor combined with a smart Lock and potentially door sensors.

John, I would like more clarification on your thoughts of "one option that I'm considering is using a new smart garage door sensor..." Is this in lieu of using any proximity devices? 

Not necessarily.  You just want any solution that is reliable.  Unfortunately Samsung's implementation with the geo-fence on the phone hasn't been for me and least not on Android (Galaxy S8+), not sure on iPhone.  But that's not at all intended to discourage you.  Other proximity sensors out there may work, so it's worth the research.  if you can find a reliable proximity sensor of sorts, then that is your solution.  If you can't, you can probably get creative by using a combination of sensors and rules.  I'm currently researching using my Nest thermostat's proximity sensor, but I just got it so haven't tested it much.  Also considering the smart lock + smart garage door opener + old adt door sensors with rules, as mentioned above

I just reread your original post and saw that you already have the ST hub.  Setup a rule in the ST app to send you a text message when your phone is away and then another text when your phone returns.  Then just test it out for a few days and you'll see how reliable is for you.  If it works well, go with that approach.  If not, I'd still recommend the project (just ordered a Konnected board myself), but I'd just come up with a different plan for detecting that you're away/home.