The officer was said to be shaken by the find, which was made at Shandon Park Golf Club, close to the Police Service of Northern Ireland (PSNI) headquarters in Belfast on Saturday morning (June 1).
Army bomb disposal experts said it was a viable improvised explosive device that failed to detonate.
One of the vehicles PSNI officers believe was used by the offenders had a Republic of Ireland registration plate.
Detective Superintendent Sean Wright said the investigation centres on two cars, one of which was a Green Skoda Octavia with a Dublin registration, 01 D 78089. The other is a silver Saab with the registration NFZ 3216.
Both cars were subsequently abandoned and burnt out in Etna Drive in north Belfast during the early hours of Saturday morning.
Source: Police Professional