I've lost my keys exactly twice. The first time happened when I was taking part in a go tournament in another city. I was tired and took a nap on the couch of the club hosting the tournament. My key ring with all my important keys attached to it had slipped out of my pocked and had ended up inside the couch. It took a couple of hours to find the keys after I noticed they were lost. I scoured several pubs and other locations that I had been to that evening. The key ring had my car keys in it, too, so I would've had to take a train home and come back with the spare key with a parking ticket waiting for me the following day.
I did not learn my lesson, however. The next time I lost my keys was many years later. They probably fell out of my pocket again. It snowed heavily that day. I did not find the keys where they must have fallen because a snow plow had pushed them away with the snow. We lived in a rental apartment those days and it would've cost us about €850 to pay for the locks of the apartment and a garage to be changed.
I did recover them a couple of months later when I went to the police station to ask for them. One of the keys was bent out of shape, which must have been the work of a snow plow. This time I learned my lesson and immediately purchased a chain which I always keep attached to my jeans or my shorts. Losing keys is a major headache and I never intend to do it again.
By the way, I agree with your post. De-cluttering helps a lot.