We were sitting in a rooftop restaurant, 30 storeys up, overlooking the Empire State building in New York, when my daughter confessed that she thought she was being spied on by a professional network of cyberspooks.

‘Look at this,’ said Lois, presenting me with her smartphone, where an advert for a snazzy little instamatic camera was displayed. It had popped up a few seconds earlier, when she’d logged on to Instagram.

She met my quizzical ‘so what?’ face with exasperation.

‘What were we talking about? Just now? In the street, down there?’ she said.

Picture perfect: Jen Lewis (left) and the alarmingly similar advert sent on Facebook shortly after

Sure enough, we’d been window shopping before our lunch reservation, and spotted a little gadget shop. I remembered Lois had commented on the instamatic cameras on display (dropping a few hints for her forthcoming 21st birthday, I suspected).

We’d had a brief conversation about how they were all the rage in the Eighties, and how one of my memories of Christmas parties at my parents’ house was listening to that familiar ‘whirrr’ and watching the wealthier guests flapping about the instant photos, as everyone waited for them to dry.

They were the selfies of their day, and good fun (if you could afford the camera film). How lovely that they were making a comeback, I commented. And we moved on.

Then, less than 20 minutes later, an advert popped up on Lois’s phone, for the exact same product. Same colour, same model, same everything.

‘They’re listening, they’re watching,’ she said.

‘Oh don’t be daft,’ I replied. ‘Who’s listening? Who’d want to listen to us?’

‘I’m serious,’ said Lois. ‘This keeps happening. This is no coincidence. Someone is listening to our conversations. Advertisers. They’re listening via our phones’ microphones.’

Our activity on websites and apps and demographic information is gathered using increasingly sophisticated technology to bring us personalised adverts (stock image)

A little melodramatic and paranoid, you might think. I certainly did. I assumed Lois had simply been researching the product online before we flew to New York, and had forgotten.

We all know ‘targeted advertising’ has been prevalent for some years now, via our social media apps and search engines. Facebook was one of the first to introduce it four years ago. It’s no big secret: go on the John Lewis website and choose a blouse, or Google Nigella’s smart eye-level oven, and the next time you log on to Facebook or Instagram, there’s a good chance they’ll pop up as adverts there.

While it felt a little uncomfortable and intrusive to begin with, we’ve all sort of got used to it.

Our activity on websites and apps and demographic information is gathered using increasingly sophisticated technology to bring us personalised adverts.

People’s electronic markers — known as ‘cookies’ — from websites they visit are gathered and passed to advertisers so they can target us with products relevant to our tastes and interests (and ones we’re more likely to buy).

Facebook categorically denies it uses smartphone microphones to gather information for the purposes of targeted advertising

It is not illegal. Although under the Data Protection Act 1998, a person has to actively consent to their data being collected and the purpose for which it’s used, few people actually take time to police what they consent to.

The terms and conditions and privacy statements you sign up to when you buy a smartphone or download an app are rarely scrutinised before we tick the box and wade in.

But Lois swore she hadn’t Googled an instamatic camera. That was the first time she’d ever had a conversation about them. ‘I’m telling you, they’re listening,’ she said, and I admit I stuffed my own phone a little deeper into my bag. Could she be right?

Well, hundreds of other people seem to think so. Stories on Twitter of these ‘blind coincidence’ adverts are abundant.

And not just restricted to voice snooping either — some are convinced their phones are spying on them via their cameras, too.

Last month, a creepy story swept social media about an American woman called Jen Lewis who was shown an advert on Facebook for a bra — featuring a model wearing exactly the same clothes she was wearing at that moment. The same pink shirt and skinny jeans.

Lewis, a writer and designer, recreated the model’s pose and posted the near-identical pictures side-by-side on Twitter where they went viral with more than 20,000 likes.

While Facebook insisted the ad was a coincidence, hundreds of horrified social media users commented — many suggesting the ad could have been targeted with image recognition software, using Jen’s laptop or smartphone camera as a spy window into her life. ‘Seriously, cover up your camera lens,’ warned one, as stories were swapped of people receiving adverts for wedding planners, minutes after popping the question, and cat food after merely discussing whether to buy a cat.

People’s electronic markers — known as ‘cookies’ — from websites they visit are gathered and passed to advertisers so they can target us with products relevant to our tastes (stock image)

One Facebook user is so convinced his conversations are being monitored that he switched off the microphone on his smartphone — and, sure enough, there haven’t been any more ‘strange coincidences’ since.

Tom Crewe, 28, a marketing manager from Bournemouth, was immediately suspicious in March when he noticed an advert on Facebook for beard transplant surgery. Only hours earlier he’d joked with a colleague about them both getting one, as they remained smooth-faced, despite their age.

‘I had my phone’s Facebook app switched on at the time. Within a few hours, an ad came through for hair and beard transplants,’ he says.

‘I just thought: “Why have I been targeted?” I’d never Googled “hair or beard transplants” or sent an email to anyone about it or talked about it on Facebook.’

The fact that the ad for beard transplants was so unusual and specific made him suspect his phone had been eavesdropping.

He became convinced when later that month he received an advert to his phone — again weirdly and quite specifically — for Peperami sausages.

Companies have developed algorithms that can look for patterns and determine potentially useful things about your behaviour and interests (stock image)

‘Again, it was a casual conversation in the office. I’d just eaten a Peperami, and it was a few hours before lunch, and a colleague joked how he didn’t think this was a particularly good thing to have for breakfast.

‘Again, I’d never Googled the product or mentioned it on Facebook or anywhere online. It’s just something I buy during my twice-a-week shop at Tesco.

‘Then I get an advert for it. This happened within two weeks of the beard incident.’

It so disturbed him that he researched it and saw others talking about it.

‘I saw articles and got information and turned off the Facebook app’s access to my phone’s microphone. I’ve not noticed it happening since then.’

Facebook categorically denies it uses smartphone microphones to gather information for the purposes of targeted advertising.

A spokesperson said being targeted with an advert for a beard transplant was just an example of heightened perception, or the phenomenon whereby people notice things they’ve talked about.

With 1.7 billion users being served tens of adverts a day, there’s always going to be something uncanny. Google and WhatsApp also categorically deny bugging private conversations, describing the anecdotal evidence as pure coincidence.

One thing technology experts agree on, though, is that the ability to create technology that can randomly sweep millions of conversations for repeated phrases or identifiable names, definitely exists.

Companies have developed algorithms that can look for patterns and determine potentially useful things about your behaviour and interests. Whether they are being used by the companies with access to your phone, however, remains unproven.

Not convinced? Consider the Siri or Google Assistant functions, designed to understand your voice and pick out key phrases, and with a huge vocabulary in their grasp.

I saw articles and got information and turned off the Facebook app’s access to my phone’s microphone. I’ve not noticed it happening since then

It’s not too big a stretch to think of this technology developed to sweep conversations as a marketing tool. ‘Smartphones are small tracking devices,’ says Michelle De Mooy, acting director for the U.S.’s Democracy and Technology Privacy and Data project.

‘We may not think of them like that because they’re very personal devices — they travel with us, they sleep next to us. But they are, in fact, collectors of a vast amount of information including audio information. When you are using a free service, you are basically paying for it with information.’

As yet, however, there’s no concrete evidence that we are being listened to. Any complaints about spying would be dealt with by the Information Commissioner’s Office (ICO), which handles legislation governing how personal information is stored and shared across the UK.

They say no one has complained officially. Tales of cybersnooping haven’t gone beyond ‘shaggy dog stories’ on Twitter and Facebook.

When approached by the Mail, an ICO spokesman said: ‘We haven’t received any complaints on the issue of Facebook listening to people’s conversations.

‘Businesses and organisations operating in the UK are required by law to process personal data fairly and lawfully, this means being clear and open with individuals about how information will be used.’

That law, however, is struggling to keep up with technology, according to Ewa Luger, a researcher and specialist in the ethical design of intelligent machines, at the University of Edinburgh. ‘I think this is a problem ethically,’ she says. ‘If I had an expectation that this application was recording what I was saying, that’s one thing, but if I don’t, then it’s ethically questionable. I may be having private conversations and taking my phone into the bathroom.

‘This is a new area of research — voice assistance technology. We have only been looking at this for 12 months. It takes a while for research to catch up.’

In the meantime, Lois and I have turned off our microphones. It’s easy to do via your phone’s Settings.

To be honest, I don’t think there are people with earphones in a bunker, desperate to know what car I’m thinking of buying, but I’d rather, in this increasingly public world, maintain a bit of privacy. You really don’t know who’s listening.

■ Additional reporting Stephanie Condron