Cybersecurity specialists say microphones may be constantly picking up clues about someone

Have you ever had the uncomfortable feeling that your smartphone is eavesdropping on you?

The eerie truth is that it may be doing just that — to show you ads tailored to your interests, experts say.

Nearly half of Britons believe they’ve fallen victim to ‘sonic snooping’, where ads for a product pop up on your phone screen shortly after you’ve talked about it or seen it on TV.

Cybersecurity specialists NordVPN have now said this phenomenon is really happening, thanks to apps that listen in on background noise – and possibly private conversations as well.

They said a phone’s microphone can constantly pick up clues about where someone is, what they’re doing and what they’re interested in.

A survey of more than 1,000 people in the UK conducted by the company found that 45 per cent had noticed an ad for a product or service they hadn’t searched for but had recently talked about or seen on TV.

Half of the respondents said it made them feel they were being followed, while one in eight said it scared them. Still, two-thirds of those affected said they had no idea how to prevent it.

Much of the data is collected through a controversial form of digital monitoring called ultrasonic cross-device tracking.

Here, the smart products in our home – from TVs to laptops – can keep an eye on us by secretly communicating with each other.

They do this by sending ultrasonic waves – which are too high for humans to hear – through the room that reveal both our location and what we are up to.

Our smartphones, laptops and tablets then pick up these signals through their microphones and use the data to show us relevant advertisements when we are on social media or browsing the web.

This is why so many apps ask for user permission to access the microphone – even if it seems irrelevant to its purpose.

Cybersecurity consultant Adrianus Warmenhoven said cross-tracking was a goldmine for advertisers, allowing them to “gather a lot of information about you, all without your knowledge.”

Much of the data is collected through a controversial form of digital monitoring called ultrasonic cross-device tracking

But he said it wasn’t possible to prevent your smart home devices from emitting these ultrasonic waves, even when they’re not connected to the internet.

Instead, the only way to protect against unwanted tracking was to check what “permissions” users granted to the apps they installed, specifically whether they had access to the microphone. He also suggested that smartphones could collect data about listening to our private conversations, though the evidence so far is only anecdotal.

In one of the company’s experiments, employees successively discussed topics no one had ever shown an interest in, with their phones within reach.

One of the participants, Jason, talked about buying a new Volvo. Despite never owning a car, showing no interest in it or searching for the brand online, he was inundated with Volvo advertisements.

Mr Warmenhoven said: ‘It’s hard to say how often smartphones eavesdrop on your private conversations, but technology has certainly replaced it.’