One night in 2023, Eric was scrolling on a social media channel he regularly browsed for porn. Seconds into a video, he froze.

He realised the couple he was watching - entering the room, setting down their bags, and later, having sex - was himself and his girlfriend. Three weeks earlier, they had spent the night in a hotel in Shenzhen, southern China, unaware that they were not alone.

Their most intimate moments had been captured by a camera hidden in their hotel room, and the footage made available to thousands of strangers who had logged in to the channel Eric himself used to access pornography.

Eric (not his real name) was no longer just a consumer of China's spy-cam porn industry, but a victim.

So-called spy-cam porn has existed in China for at least a decade, despite the fact that producing and distributing porn is illegal in the country. In the past couple of years, the issue has become a regular talking point on social media, with people - particularly women - swapping tips on how to spot cameras as small as a pencil eraser. Some have even resorted to pitching tents inside hotel rooms to avoid being filmed.

Last April, new government regulations attempted to stem this epidemic - requiring hotel owners to check regularly for hidden cameras. But the threat of being secretly filmed in the privacy of a hotel room has not gone away. The BBC World Service has found thousands of recent spy-cam videos filmed in hotel rooms and sold as porn, on multiple sites.

Much of the material is advertised on the messaging and social media app Telegram. Over 18 months, I discovered six different websites and apps promoted on Telegram that claimed to operate more than 180 hotel-room spy-cams.

One of the most prominent spy-cam porn traders I came across was an agent known as AKA. Posing as a consumer, I paid to access one of the livestreaming websites promoted by him.

During our 18-month investigation, we identified about a dozen agents like AKA. The exchanges they had with subscribers made it clear they were working for others higher up the supply chain, who they referred to as camera owners. What is clear is there are significant sums of money to be made. The average annual income in China last year was 43,377 Yuan, but AKA alone has earned at least 163,200 Yuan ($22,000) since last April.

Eric and Emily remain traumatised by their experience. They try to avoid staying in hotels and wear disguises when going out for fear of being recognized. Despite the dangers of this hidden industry, its growth continues unabated, posing a persistent threat to individuals seeking a sense of privacy.