Women are being warned to be more cautious as spy gadgets used by tech-savvy peeping Toms are becoming more sophisticated and obscure.
The warning comes after a camera encased in a wall-mounted plastic coat hook was reportedly found inside the Ma On Shan MTR station's female staff toilet earlier this month.
Photos of the device began circulating on mobile chat network WhatsApp last week, but the MTR Corporation said it had received no complaints or reports about the matter.
"The situation is becoming quite worrying and women need to be on the alert," said Dr Elizabeth Quat, who chairs the women's affairs committee for the Democratic Alliance for the Betterment and Progress of Hong Kong.
"Sometimes, photos of these victims can be posted on the internet without any censoring, and once the photos are posted online, they will remain there forever."
The DAB found the same type of device, which saves footage to an SD memory card, selling in Sham Shui Po for HK$800.
They also showed a HK$500 smoke detector, which had an HD webcam concealed inside it, that could stream live footage to a smartphone or tablet computer over Wi-fi.
Quat yesterday urged the government to introduce legislation to control the sale or use of such devices and to impose stricter punishments on people caught using them.
The party said a range of surveillance devices were being sold across the city and most could be purchased for less than HK$1,000.
From sneaky light bulbs to high-definition cameras concealed within smoke detectors, most of these devices can be easily bought online or at flea markets such as at Apliu Street in Sham Shui Po, the DAB found.
Quat urged women to be wary of lights or hooks that may have been tampered with and "strange holes in walls" in changing rooms.
Fellow DAB member Philip Li Ka-leung said that as smartphone technology advanced, secret surveillance would only get more sophisticated.
"Similar devices are bought and placed at home to monitor maids or children. This is already an intrusion of privacy," he said.
"The consumer culture and attitude towards these devices must change."
In December, an employee of a property development company was jailed for three months for secretly filming people using the bathroom at a show home with a mobile phone hidden in a paper box.
In August, a man received a two-month suspended jail term for taking videos up the skirts of women at an MTR station using a pinhole camera embedded in his shoe.
Last year, railway district police received 110 reports of women being secretly filmed from under their skirts, up from about 100 in 2012.
There have already been about 30 reports of "upskirt" violations in the first four months of this year.