Shoosh! Can you keep your voice down? Our phones might be listening in to us. You think I'm being paranoid? Then check out those online stories about people having chats to friends and family with their smartphones lying within easy earshot, only to find – what a coincidence! – advertisements later popping up on their Facebook and Instagram accounts related to the car, holiday destination or shoes they'd been talking about.

We're used to targeted advertisements appearing on our social media accounts soon after we've searched or purchased a product via Google, thanks to "cookies" that track our clicks across the web. But there is something far creepier about the prospect of the mics in our phones, which are only supposed to switch on when we utter the trigger words "Hey Siri" or "Okay Google", listening to us unbeknownst.

Last year, a US TV manufacturer, Vizio, was fined $US2.2 million for tracking viewing habits and selling the data to advertisers; Samsung caused a stir in 2015 when it warned its customers that anything they say around their new television will be "among the data captured and transmitted to a third party". But any TV with voice recognition would have this potential.

"Our phones are like tiny computers," says Professor Jill Slay, Optus Chair of Cyber Security at La Trobe University. "Many apps ask us for access to the microphone – including games like Honey Quest used by children."