5DM3, 580EX and INFO screen

In playback I had the INFO on and in some shots the +/- exposure compensation icon was displayed with a -2 designate. I did not have either flash exposure compensation (both in-camera and in-flash controls) nor regular exposure compensation set to any value other than "0". Its appearance seems random since I was shooting M, Av and Tv, using the setup indoors that also included part of the outside scene through a large window - some pics have it and others don't.

Does anyone have any ideas? I'm stumped looking through the manuals and web based info.

Thanks!