Windows logo

From Halopedia, the Halo wiki

The Windows logo can be seen on the white sticker on the back of the phone.

The Windows logo is an Easter egg in Halo 3. On the multiplayer level The Pit, go into the sword room and locate a phone hanging on the wall, bright yellow with a red error message on it stating that it is out of service. Melee it and it will break, over on the back of it, there is a small Windows logo. There is also one of these on Blackout, near the basketball hoop.