I have almost lost my keys - twice.
The first time, I had taken a car service to the airport directly from the office. Before going through security, and an inventory of the items that needed to go in the bin revealed that my keys were not on my person. Ultimately, a phone call confirmed that they had fallen out of my pants pocket, and onto the back seat of the car. I would not need then until I returned a week later. Luckily, the driver was able to drop them back off at my office, where I was able to retrieve them.
More recently, I haven begun commuting via train every day to NYC. As a matter of course, I keep my keys in my bag. One evening, just before my stop on the ride home, I took them out of my bag and laid them on the seat, anticipating that I will pick up the keys, and then use them to open my door in a few minutes. I get to my door, and I do not have my keys, they are still on the seat. Once again, luck was on my side. Another passenger found them, was able to contact me through a key tag, and so I retrieved the keys a short time later.