Police in the western city of Cologne responded after the incident at the weekend in which the robot's suspected 16-year-old owner and a 19-year-old companion were said to have filmed the alleged assault.
They were said to have streamed the incident and commentated on it online.
Cologne police spokesman Christoph Bermel said the two male teenagers have not been charged but are considered suspects, and investigators were examining whether the robot's movements were preprogrammed or controlled in real time.
Police were unfamiliar with this type of incident, he said.
"We have not been aware of this phenomenon so far," Bermel told The Associated Press, saying it was "new territory for us".
Emergency teams that were dispatched to the scene near a fast-food restaurant found the robot in the boot of a vehicle, Cologne police said in a statement on Monday. The robot and the two young men's mobile phones were confiscated.
Videos circulated on the internet show two young men escorting the robot toward a group of women, bumping into one and hugging others, on a lively nighttime street.
One of the men repeatedly shouts at the robot and pulls it backward after it makes sexual gestures, prompting laughter among people in the area.
On the police's Facebook account on Tuesday, officers criticised inappropriate comments and warned that sharing or joking about such incidents can worsen the impact on any victim.
