Like Jay I also have a wall keypad where pressing it turns out all of the downstairs lights last thing at night. For us it's at the bottom of the stairs so we just press it on the way past up to bed.
That may not be the exact right answer for you but might be a useful alternative way to think about it - I.e. Instead of thinking of something you do every night and trying to detect it, instead thinking of something or things that need doing every night that could be usefully automated to add enough value that you get in the habit of manually triggering that and can tie the sleep mode to it. For example, mine also checks door/window sensors for all of the downstairs at this point and if any are open, leaves the lights on and uses speech to announce which are still open so we can go round closing. That's useful enough that it just becomes a habit to press the button on the way past each night.
Jon