I have recently been using a particular 2-step authentication system and there is something odd about the SMS messages I am sent which I have never seen before.
When I receive and SMS from the system, there is no notification, and the message does not immediately show up in my hangouts inbox. Instead, the SMS pops up as an alert dialog over whatever I am doing with the phone at the time with the option to "Cancel" or "Save". If I tap "Save", the message is then stored in hangouts and I get a notification.
What I would like to know is why this happens. Is there a (rarely used) part of the SMS specification which provides this functionality? Is this 2-step auth system making use of that? Perhaps setting a flag in the header or something?
I am running Android 5.0.2 and the default SMS app is Google Hangouts. I'd like to be clear though that this only occurs when receiving texts for this particular 2-step authentication process. All other messages are received as normal.