At the time of the seizure, bomb disposal teams had found six boxes of explosives in the fuel tank of the truck but the further discovery only came on Thursday.
"While trying to move the decaying truck from the car park, boxes of TNT fell off the chassis," a police statement said.
It said the concealed explosives had gone unnoticed by the army bomb disposal squad.
Separatist Tamil Tiger rebels used their trademark suicide bombings to spread fear in Sri Lanka's capital and other key towns, but were defeated in a major military offensive in May 2009.
The conflict cost the lives of some 100,000 people between 1972 and 2009.Since May 2009, there have been no attacks blamed on the Tamil Tigers, but authorities have warned that remnants of the separatist rebels could try to regroup.