Recently, we were visited by my sister-in-law. We ended up talking about a topic, say Topic X. Topic X _never_ comes up when I and my wife are alone; it is simply not a part of our life. We have been married for 15 years, without talking about it even once.

However, it did come up while talking with my s-i-l, and my Android phone was lying nearby. Minutes later, after my sister in law had left, I was presented a Topic X - related ad on YouTube.

It was displayed only once, I ignored it, and no Topic X - related ad was shown after that.