Yandex spokesman Ilya Grabovsky said that some Internet users contacted the company Wednesday to say that its public search engine was yielding what looked like personal Google files.
Russian social media users started posting scores of such documents, including an internal memo from a Russian bank, press summaries and company business plans.
Grabovsky said Yandex has alerted Google to the concerns.
It was unclear whether the files were meant to be publicly viewable by their authors and how many there were. Google did not comment.
Grabovsky said that a Yandex search only yields files that don't require logins or passwords. He added that the files were also turning up in other search engines.