I think this is a bug. I'm currently using Sublime Text 2 Build 2216 on Mac OS X. It's saving a modified file every time it gets the focus back. Say, I'm editing some file, now I switch to another file on another tab and get back to the previous file. Boom! The file gets automatically saved.
I checked my "save_on_focus_lost" and it is set to false. I even copied that parameter to my User settings to force this option but nothing changed. However, this is not the same. Having this parameter enabled, makes Sublime to save the file on "focus lost", not "focus re-gained".
Please note that this is happening since some versions ago, although I don't remember which exactly, sorry.
This is also happening if focusing on another application and returning to Sublime, or even pressing super+shift+p (to access quick panel) and closing it. In summary, the file gets saved everytime it re-gains focus; no matter the reason that made it loose the focus.
EDIT: Some typos.