>Since then, every time I've used the key, I force myself to see where it is and to do so immediately.

This is a good lesson - pain helps remind us. Someone who has lost money out of their pocket, always remember the even whenever they put more in. 
This is the first time I have lost my keys and I still think they will turn up somewhere. We have a lot of stuff around at the moment in the house, so it could be anywhere, Unless it slid out of my jacket pocket while getting in the car somewhere....