What driver are you using? All the community offerings I'm aware of have some custom attribute that can help you figure this out, then it's just a matter of using a rule or some app to notify you or whatever you want.
Isn't there still a "last check-in"-type attribute? That can be used.
A link would be great, but if my guess above is correct, this is a custom attribute you could use with, say, a Rule Machine rule. Trigger at either a specific time each day and see if the value is farther away than you want at that time (I'd use the Unix/epoch time option to make that easier), or even just trigger on that attribute changing and staying that way for so long, then notify.
I'd probably do something like the latter (or a third option, like the former but wait to notify) so I have control over when the notification happens. It's possible the app you found has an option for this, too, but I no longer use it.
If you do write a rule to check for the devices going 'not present' you might want to put a delay before sending a notification as sometimes they can 'drop off' and go not present but come back on line again.
Just something to bear in mind and save needless notifications.
EDIT: This is something I have written in webCoRE.