During the Credits
Environmental shots of the Halo universe are shown in the background during the credits of Halo 5: Guardians.
After the Credits
There is a stinger after the credits of Halo 5: Guardians, however, the game must be beaten on the Legendary difficulty setting in order for the scene to be unlocked. There is no stinger after the credits when beating the game on the other modes. Read more details involving spoilers below.
In the extra scene, another Halo is shown as a re-animated Cortana can be heard humming off-screen.
Special thanks to Chris Stanford for the stinger submission!