Part of the problem is related to my lack of interest in driving, as the last few trips I made out of the house before losing the keys were while my wife drove. Thus, a span of about three days existed between the times that I definitively knew my keys were missing and when I knew that they were gone.
I hit upon an idea and carried it to fruition that helped solve the mystery today, though. I announced to my visiting children that anyone who found the keys would be the recipient of $25 in reward funds, and within 10 minutes my youngest daughter - who paused from her participation in a live chat - found the keys in the couch cushions. I would have sworn that I thoroughly searched that couch, but found is found.
Besides, I would have spent four times that amount of money in getting new keys and locks made, since three of the keys were for one of my employers, and they charge a hefty fee to replace keys lost by careless employees. Add to this the peace of mind in knowing that my keys have not fallen into criminal hands, and I say that $25 is a small price indeed to pay.