To locate it, proceed through the level to the large gondola with the first Enforcer. Destroy the Enforcer, and then deactivate the four Plug locks holding the gondola in place. This reveals a holopanel in the middle. In the center of the panel is what appears to be a small image of a six-fingered hand.
There is contention as to which configuration the Forerunners may have sported. One camp states that the webbing on the icon indicates the last digit is a finger, not a thumb, or that a second thumb would not be as useful as a wider hand. Another interpretation posits that the icon isn't a literal depiction of a hand, but indicating that the icon was ambidextrous and could be used by the left or right hand. If so, then it is a replica of a human hand, but the device itself is meant to be activated by either a human-shaped left or right hand, giving further insight to the Forerunner-Human relationship. The icon could also be heavily stylistic, and not meant to accurately show a Forerunner hand, much like how real world signs will have simple representations of humans, i.e. stick figures.