Footage on China's state-run CCTV showed crew members poking into the water a device shaped like a large soup can attached to a pole. It was connected by cords to electronic equipment in a padded suitcase.
"If the Chinese have discovered this, they have found a new way of finding a needle in a haystack," said aviation expert Geoffrey Thomas, editor in chief of AirlineRatings.com. "Because this is amazing. And if it proves to be correct, it's an extraordinarily lucky break."
There are many clicks, buzzes and other sounds in the ocean from animals, but the 37.5 kHz pulse was selected for underwater locator beacons because there is nothing else in the sea that naturally makes that sound, said William Waldock, an expert on search and rescue who teaches accident investigation at Embry-Riddle Aeronautical University in Arizona.
But after weeks of false alarms, officials were careful Sunday not to overplay the development.
"We are hopeful but by no means certain," Australian Prime Minister Tony Abbott said. He added: "This is the most difficult search in human history. We need to be very careful about coming to hard and fast conclusions too soon."
A senior Malaysian government official said Sunday that investigators have determined that Flight 370 skirted Indonesian airspace as it flew to the southern Indian Ocean.
The official, who spoke on condition of anonymity because he was not authorized to talk to the media, said Indonesian authorities confirmed that the plane did not show up on their military radar. The plane could have deliberately flown around Indonesian airspace to avoid detection, or may have coincidentally traveled out of radar range, he said.
Houston, the search coordinator, said there had been a correction to satellite data that investigators have been using to calculate the plane's flight path. As a result, starting on Monday, the southern section of the current search zone will be given higher priority than the northern part.
The signals detected by the Chinese ship were in the southern section, Houston said.
Up to 12 military and civilian planes and 13 ships took part in the search Sunday of three areas totaling about 216,000 square kilometers (83,400 square miles). The areas are about 2,000 kilometers (1,200 miles) northwest of the Australian coastal city of Perth.