A fix for this problem has been seen on a new version of the firmware shipped out with the latest version of the Nokia Lumia 620. While the current Lumia 620 model mostly has the 1250 firmware, a firmware version numbered 1307 seems to have the repair. With a new feature called Storage Check, users can clear temporary cache files in the "other" category and recapture the native storage that has been disappearing. The same Storage management is found on firmware 1308 for the Nokia Lumia 920.
Eventually, the new firmware versions containing this updated Storage management feature will have to be rolled out by Nokia. After all, no one likes to have even 1GB of storage hijacked by a mysterious file.