The device was taken to the Stockley Park area and safely detonated shortly after 3pm.
The Porters Way site, now a housing development called Drayton Garden Village, comprises nearly 800 homes.
Police say the suspect device was found in the Parkwest area of the site, and 'all the relevant safety precautions' were taken.
Eyewitness Kelly Gentles of Porters Way told the Gazette: "The first thing I saw was the police tape.
"We walked up to the shops but [the police] sent us back."
She said large Land Rover-type vehicles which she assumed were driven by the bomb disposal squad officers then arrived.
She said officers were on site for about 30 minutes.