"proof" is not to be confused with proof, or evidence. "Proof" is any credential you can provide to show that you(1) have been at the geohash(2), on the correct date(3). Perfect "proof" tries to provide credentials for the three parts enumerated here, usually in the form of pictures of your stupid grin and your GPS displaying coordinates and a time. Notwithstanding, less than perfect "proof" is perfectly acceptable if it is backed by a good story, a decent excuse or your willingness to account for a lazy or just unlucky geohasher. Some of the best expeditions will leave you with no "proof" at all, just because you forgot those batteries, the SD card, or because you were trampled by a herd of meese. This is the time to remember that a good story can make up for anything and invest some extra time in your report.