Most recently, a source described a "dark grey" or "gunmetal" colored notebook of slender proportions making the rounds in an area where Apple traditionally conducts stress tests.
Though noticeably thinner but otherwise similar in size to the company's 13-inch MacBooks, the notebook appeared to have a metallic sheen to it, which set it apart from the current black MacBook.
This most recent sighting seems to fall in line with earlier reports that a slender pair of notebooks were making their way around Apple's campus, clad in both traditional and black aluminum enclosures.
Still, there has been no definitive evidence to link these sightings the company's sub-notebook project, which has been detailed (1, 2, 3) by well-respected sources over the past year and is due to culminate early next month with an introduction at the Macworld Expo in San Francisco.
Additionally, it should be noted that sightings of particular colors and enclosures at Apple ahead of a product's release are not always fail-safe indicators of that product's final appearance. Apple is also likely working on more slender consumer MacBooks, which could at times be confused with the upcoming sub-notebook.