If you discover a peeping Tom in your backyard, you have a right to seize his spying gear and trenchcoat and turn him over to the Police, which is what the Chinese Navy did.  As I posted earlier, the Americans were likely spying on China.

"Although the ministry said the drone would be returned to the United States in a “proper way,” the statement stopped short of saying when or how the device, which Chinese and American analysts say was most likely used to gather intelligence about Chinese submarine activity in contested waters, would be returned, or if it would be handed back intact."

http://www.nytimes.com/2016/12/17/world/asia/china-us-drone.html?ref=asia&_r=0