BEIJING - Chinese dissident artist Ai Weiwei found several hidden listening devices in his Beijing studio after he returned from a trip to Germany, his friend Liu Xiaoyuan confirmed Monday.

Ai could not be reached, but he has posted photos of the bugs found in electric sockets on his Instagram account as well as a video clip in which someone set off firecrackers in a metal bucket next to a listening device. "Did you hear it," Ai wrote next to the video.

In another photo of a bug, Ai wrote: "There are always surprises."

The artist has in the past upset Chinese authorities with boldly critical remarks of the ruling Communist Party and its government. He was briefly detained in 2011 and released after 81 days in jail. The authorities withheld his passport but returned it to him four years later, in July.

Since then, the artist has toned down his criticisms of the government. He has travelled to Germany and England, but is in Beijing to visit his mother, Gao Ying, as reported by Radio Free Asia. Liu confirmed the report.

Gao told Radio Free Asia that she believed the devices were installed by the Chinese authorities four years ago when Ai's studio was ransacked by the authorities. Ai himself has been under tight surveillance since his 2011 release.

In social media, Liu said the bugs were found as Ai was renovating the studio. The devices were found in a living room, an office and a bedroom, Liu said.