I was on an overnight train between Cairo and Aswan in Egypt a couple of years ago. My passport was in my Pocket.. nice and safe. The passport of one of my traveling companions was in their backpack, on the shelf above their seat. In the morning they disocvered it gone. During the night, when we sleep, sombody had rifled through her bag and taken the passport and a few other things. (cant remember what else was taken).
The passport is the one thing you can loose when traveling that is going to cause serious inconvienence. without it you cant prove who you are. (Some non first world countries dont even ACCEPT non passport identification. so dont go trying to show off your drivers license issued in your home country.. )