A number of secure accounts have been stolen on Steam as of late. The common software components that I’m aware of are the new Steam Friends system and Team Fortress 2. One of those packages have a major security flaw allowing Steam passwords to be compromised. I don’t know if its just saved passwords or not. But I encourage everyone to lobby Valve about this issue before it happens to you (possibly again).
Update 5/31/18: Valve finally patched the exploit!