This is up there with "I was talking about X with my friend, and half an hour later I saw a Google ad for X. Ergo, Google are listening in on my phone's microphone."
Apparently, rational HN readers don't require any actual evidence before going with the assertion that "Boris Johnson uses search terms in interviews". Crazy times.
I've had this happen before, and it was just a little bit very creepy indeed, and did make me wonder.