The New Paper
Armed with small and increasingly cheap pinhole cameras – some with a lens no bigger than a five-cent coin – they prowl locker roomers and toilets.
Their target: To spy on young women.
On Saturday, The New Paper reported that a teacher allegedly installed three pinhole cameras in a girls' toilet in his school, which is in the western part of Singapore.
TNP understands the pinhole cameras were disguised as light switches and placed at waist height.
So how do you spot these tiny cameras?
Never mind if you are clueless about the advances in technology, as one of the best ways to check for cameras is to do it the old-fashioned way, said Mr Singh.
He added: "One of the things to watch out for is unauthorised people hanging out around female toilets – they could be there to fix those cameras.
"Otherwise, check for any suspicious 'dots' on the ceiling, for example. That could be a camera on you."
But other experts said these spy cameras are getting harder to spot.
Especially when they can be fitted in pens and made into buttons that can be sewn onto a shirt.
And they can also be hidden in other everyday objects.
"They can be concealed in smoke detectors, for instance," said Mr Patrick Lim, the sales and marketing director of Ademco Security Group.
Another surveillance expert said they can also be fixed to a car key or a USB flash drive, or even be built into a shoe.
These cameras, said private investigator Lawrence Koh, typically record up to four hours of video footage before running out of power and storage space.
"I've seen it built into clocks or flower pots… and the problem is these cameras are getting more difficult to detect," said Mr Koh, who owns private investigation firm S K Investigation Services.
"The truth is not all these cameras can be seen by the untrained eye because many of them are wireless… in most cases you need detectors to fish these cameras out," he added.
And they are becoming cheaper too.
A wireless hidden camera that records voice and images in colour can be bought for about $200 at Sim Lim Square. Two years ago, such a camera would have cost $400.
There are also much cheaper variants available online.
One website is offering for sale what it calls "the smallest spy camera in the world".
While it looks and writes like an ordinary pen, the camera, its sellers claim, can also record up to 16 hours of colour video without anyone knowing.
That camera retails for about US$28 (S$35).
In the recent case involving the school teacher, it is believed a student last Wednesday discovered one of the cameras disguised as a light switch.
The student alerted a school staff member, who later found two more similar devices in other cubicles in the same toilet.
TNP understands that the staff member took the cameras to the staff room and later left the room. The staff member then returned and found the cameras missing.
When footage from a closed-circuit television camera installed in the staff room was checked, it showed the male teacher walking away with the devices.
Sources said the school alerted the police about the incident the next day.
The teacher was subsequently arrested.
None of the private investigators TNP spoke to said they have ever been been approached by schools to check for hidden cameras.
And this service does not come cheap, said Mr Koh – professional surveillance cameras are more advanced, and will use frequency ranges not easily spotted by the cheaper camera detectors.
Usually, it costs between $2,000 to $3,000 to sweep an area about the size of a five-room flat, he added.
Cheaper detectors are available from stores at Sim Lim Square for less than $100.