AppleInsider is reporting that sources have spotted sightings of an unfamiliar MacBook model around Apple’s main campus in Cupertino.

The source described the colour as “dark grey” or “gunmetal” with the notebook being slender proportions making the rounds in an area where Apple traditionally conducts stress tests.

The unit is said to be similar to that of the 13-inch MacBook but appeared to have a metallic casing.

The sightings fit in with the hottest rumour surrounding the upcoming Macworld Expo. A thin ultra-portable Mac.