So the key reminder is here. It combines the function of reed switch, distance sensor, speaker and IO provided by Adafruit. It simply reminds you when you open the door but forget to take the keys. More details can be found in the Instructables link above.
It’s a box (and Allan doesn’t like it) where you can put your keys on it. No it’s definitely not the best looking thing I have ever made, but hey, it means I have a lot of space to improve in the future. And giving more time (two weeks I spent on this project, one and half of them spent waiting for parts being delivered. Good job UPS!)