I think it's possible to know that you forgot something. You can remember the title of a song, but the lyrics/details are deleted and replaced with a much shorter "memory was deleted to free space" message :-)
This is true - the metadata about a memory and the actual memory are often stored seperately. In regard songs memory loss seems to occur with the song title before the cord progression is lost. This results in the classic old person lament that all new music sounds the same.
