Other people's username/passwords saved to my keychain

When I first got my work computer (MacBook Air), I saw that it had some passwords saved to it, although the IT guy had said it was wiped. Even some passwords from people who haven't worked here for 3 years prior. I thought those people had just been previous users of this computer, so I just deleted those passwords and moved on. Recently, I've been noticing that some new username/passwords have been showing up (Facebook, Google, etc.). They're even available on my iPhone! I'm more concerned of my own accounts being available to other people who are not as ethic as I am being about this.

Does anyone know what's happening?