I lost mine briefly in Tanzania. Well, I knew where it was, but... It was in document wallet which I thought I had put into my hand luggage but it had slid between me and the opening of my hand luggage, so when I got out of the taxi, it didn't. The wallet was black, so it blended in with the seat.
You need to show your passport to enter the airport I was at, so I realised very quickly, but the taxi was gone. Other drivers saw the look of panic on my face and crowded around to ask what was wrong and how they could help. They all know each other and tried calling my driver, but he wasn't answering his phone, so one told me to jump in his car and we chased him. We caught up with him as he was on his way back to return it to me. He threw it over to me and drove off with a big wave and a smile. I had the passport, cash and tanzanite stones in there, so it would have been a very expensive loss.
I finally got in to the airport and was given a big cheer from the taxi drivers and all the airport staff. I still carry a document wallet, but one that is hideously bright and will stand out anywhere!