I did an interview with my Grandmother when I was a teenager. Yep, no real record left of the actual interview, but the facts remain. Here is how I'm planning on sourcing and describing the situation.
Near the end of her life Gertrude Sexton, Margaret Bible’s daughter, to satisfy a teenager’s budding interest in genealogy, granted some oral interviews with the author. Unfortunately, due to bad record keeping as a teenager, the original notes are temporarily unavailable.
 Gertrude Sexton (San Diego, California), interviews by David Grawrock, 1968-1970; original notes lost.
 The terminology matches the official correspondence to the author from the Immigration and Nationalization Service (INS) regarding the original documentation of his daughter’s adoption, those records have been temporarily unavailable for twenty-five years. Personally, lost is a better term, but maybe they will appear. The author assigns the same probability of the INS documents appearing as I do to the original interview notes.