Or your coworker searched for or mentioned going to Las Vegas on Facebook and you are connected on Facebook as friends, by location on netblock.
The FTC just made an announcement about SilverPush, an example of such software:
https://www.ftc.gov/news-events/press-releases/2016/03/ftc-i...
I'd even try it myself, but I don't have a facebook account, or any devices with "personal assistant" apps for that matter. For the obvious reasons.