It appears that many mobile devices, like Apple Watch, feature an always-on voice feature (“Hey Siri“) so that you don’t have to press the home button before using voice activation. You just have to say the keyword, like “Hey Siri.” That means it’s always listening to you. Always. And not just listening, but processing what you say in order to catch the trigger phrase.
My brother-in-law said he was recently talking to his wife about getting a new car. They both liked the Lincoln Navigator and talked about it, but they never started looking for one online. Never did any searches for it. They were just chatting about it in the car one day. Oddly, he started getting all kinds of online ads for Lincoln Navigators.
Siri was clearly listening and passed that information on to the Digital Advertising Alliance Network.
Many people have reported similar stories (see here and here). A 2012 article in Fast Company explains the science behind the technology, specifically mentioning the ad implications, noting that it was one to two years away.
Suffice to say, allowing this technology by your side 24/7 is just stupid. Disable it (hopefully that actually disables it).