Maybe they are misreporting when they say Radar. Could it be the magnetic anomaly detector on the P8 finding something?
Hmm, just read that the Navy scrapped MAD on the P8 to save weight. So scratch that theory.
The 777 is a plastic/carbon fiber airplane with a little aluminum. It is not magnetic nor good at reflecting much energy back at a radar/sonar source. Resting in chunks on an ocean floor it's practically invisible.