I spent a couple of fairly rough days looking (successfully, as it turned out) for a young lady who had been collecting firewood for her group's picnic campfire. We found her several miles and a few thousand feet below her point last seen after an extremely dark and stormy night had passed. She did not have her pack, or much of anything else with her.

one could easily lose their pack in a stream crossing gone bad, or in exiting a helicopter/plane "hard landing". Firefighters retreating from a flareup have been known (rightly) to jettison all their gear