Filipino fishermen found several buoys with Chinese markings near the disputed Scarborough Shoal and towed the devices back to shore northwest of the capital, Manila, Philippines officials said on Sunday.
China seized control of the rocky outcrop in the South China Sea in 2012 after a three-month stand-off with Philippine coast guard ships, preventing Filipino fishermen from getting near their traditional fishing grounds.
The shoal is about 125 nautical miles (230 km) west of the Philippines.
"Yes, there are buoys there," said Desiree Edora, mayor of Masinloc town, which has jurisdiction over Scarborough Shoal.
"I already sent the chief of police to investigate the buoys," she told Reuters.
China claims almost the entire South China Sea, believed to be rich in energy deposits, where about $5 trillion in ship-borne goods pass every year. Brunei, Malaysia, the Philippines, Vietnam and Taiwan also have conflicting claims in the area.
Beijing has become increasingly assertive in the South China Sea. Rapid reclamation around reefs in the Spratly archipelago in particular has sparked concern, both in the region and in the United States.
On Saturday, nine Filipino fishermen said they had found three long, orange containment booms, used to control oil spills, floating about six miles (4 km) off Scarborough Shoal. It was the first time fishermen there had found such devices.
"The buoys have Chinese markings," one of the fishermen told the GMA television network in the area. "The markings showed the company that manufactured it. It even has phone numbers."
The fishermen said they towed the buoys back to Masinloc to show officials there and turned them over to a coast guard detachment. They said they did not know why the buoys were there because there was no sign of any oil spills.
In late May, Reuters reported that the Philippine navy had found buoys in the disputed Reed Bank, where Manila has awarded a contract to an Anglo-Filipino consortium to explore for oil and gas in the area.
A steel marker was gone but some other buoys remained when the navy returned in mid-June.
Drilling in the Reed Bank was suspended early this year due to an ongoing arbitration case in The Hague. Manila wants to wait for a ruling before proceeding with the exploration work.
On Wednesday, China called on the Philippines to withdraw the case and return to bilateral negotiations.