Modification, perhaps? Have a buzzer where if you're out, someone can leave a note? Or a guest log or something.
(Since at least over half the time the knocks are misclicks for me, I'm not one hundred percent sure a doorknocker log would be effective? That may just be me.)
"at least over half the time the knocks are misclicks for me" .. THIS.. i know my neighbours already hate me for hitting enter on butterflies when the butterfly has just flow up and away from me.