Several users of Google's online word processing service, Google Docs, were surprised on Tuesday when their documents were blocked for violating its policies.

Users reported losing access to wedding vows and academic research, receiving messages saying that the documents had violated its terms of service. The problem was reported by dozens of people on Google's product forums.

While Google quickly restored them, blaming a botched update that mistakenly locked users out of the documents, the episode revealed what many people did not realise - Google is monitoring the ostensibly private files stored in Docs.

While the internet giant has been under pressure to better monitor the videos uploaded to YouTube, erase illegal content from its search results and better police its advertising network, few will be aware that Google Docs is being monitored. Docs is not only used by consumers but by thousands of businesses around the world, with files containing potentially sensitive information.

"This kind of monitoring is creepy," wrote Rachael Bale, a journalist at National Geographic. "I really think we need to consider how much we've all been just dumping in Google Docs," wrote Kelly O'Mara, a writer.