There is an annual portrait competition in Sydney called Head-On. This year one of the accepted portraits was of a stark, bare, empty room in a hospice (apart from the incongruous full length wallpaper on one wall of a chocolate box forest scene) where the photographer's grandfather had died. There was an interesting discussion about what makes a portrait during a floor talk I attended a few weeks ago. Sorry I don't have the answer! The organisers also said they had an interesting email exchange with one phtographer who insisted his insect "portraits" be considered.
I must admit as I went around the walls the first time it didn't register that there was no person in the photo. The power of caption and context.