[Gee101]

Could it be that they have the info of the holiday from your colleague. They then track the proximity between the two of you and then display you ads on things he is interested in.

You only notice these ads when it matches what you spoke about.

[fancythat]

Well, of course I cannot be 100% sure, but I usually don't see his other interests popping up on my ads. I must say that I am the person that doesn't use ad blockers or similar tech, as I want to see what is getting advertised on which content, etc. so I think that I am more aware than the average person of what I see and why. The holiday thing was extremely specific, it is a very small (<500 people) town in a very specific location, so unless you were not listening to our conversation and its context, it would be a very lucky guess to connect the needed dots (edit: grammar).