As far as I know, all devices connected to a WiFi network share a common public IP address. That address can be used to target all of the devices connected to that network. If you don't have any accounts set to you computer, google or whatever, may be sending you adds based on other's preferences. So it's possible no-one was sitting at your computer but you can check the browsing history. And if it helps you sleep at night, maybe it's not your co-workers, maybe it's just hormonal (pre)teens. Also, I too recommend using ad blockers if this continues to be a problem.
