Well it usually is detected by laying on a surface without movement for too long (the phone has sensors that detect motion, that's why the screen turns to the side when you do.)

If you have an apple watch then it will also do this when you walk away too far without your phone. (Once bluetooth disconnects)

So if its just an iphone, once that no movement is mixed with you being in a (I'm assuming) public place. Those two factors have apple wait till a time threshold for lack of motion and boom notification