Google said it "categorically" does not use what it calls "utterances" - the background sounds before a person says, "OK Google" to activate the voice recognition - for advertising or any other purpose. It also said it does not share audio acquired in that way with third parties.
Its listening abilities only extend to activating its voice services, a spokesperson said.
It also states in
its content policy for app developers that apps must not collect information without the user's knowledge. Apps found to be breaking this are removed from the Google Play store.
Facebook also told the BBC it does not allow brands to target advertising based around microphone data and it never shares data with third parties without consent.
It said Facebook ads are based only around information shared by members on the social network and their net surfing habits elsewhere.
Other big tech companies have also denied using the technique.
Coincidence
There is of course also a more mathematical explanation - the possibility that there is really no connection at all between what we say and what we see.
Mathematics professor David Hand from Imperial College London wrote a book called The Improbability Principle, in which he argued that apparently extraordinary events happen all the time.
"We are evolutionarily trained to seek explanations," he told the BBC.
"If you see a sign you know is associated with a predator you run away and you survive.
"It's the same sort of thing here. This apparent coincidence occurs and we think there must be explanation, it can't be chance. But there are so many opportunities for that coincidence to occur.
"If you take something that has a tiny chance of occurring and give it enough opportunities to occur, it inevitably will happen."
People are generally more alert to things that are currently occupying them, such as recent conversations or big decisions like buying a car or choosing a holiday, he added.
So suddenly those sorts of messages stand out more when they may have been in the background all the time.
Beautiful
Prof Hand is not immune to the lure of coincidence himself.
When his book was published another author published a very similar title at the same time. The author of The Coincidence Authority, John Ironmonger, shared the same birthday as Prof Hand and was based at the same university as his wife.
"These sorts of things happen," he said.
"Just because I understand why it happened doesn't make it any less beautiful."
BBC news.
