Today I walked into the bathroom at our local Winco to this exact situation. Except I hadn't been warned to assume there was a 2nd woman in the bathroom! I walked over to the stall, the handicapped one which any woman with children will agree is the Hilton of bathroom stalls, noticed that the lock was broken and could easily be unlocked from the outside, proceeded to unlock it, and got the shock of my life. There WAS a 2nd woman in there! She screamed, "Ex-cuuse ME!", and I hurried and hid myself and child in other stalls IMMEDIATLY. Ok, well first I rambled something about having seen the small ones crawl out from under the door (which btw I would NEVER allow my children to do, GROSS!).
I don't think I've been that embarassed in a long time. The worst part for me, as an oldest child, was that I KNEW it was MY fault! She had every right to be upset with me. Only, I didn't give her the chance to express it. I took my time in my own safe little stall... until I noticed my 5yr old had come out of her stall. So I rushed to get out thinking how they were now thinking I was a bad mom for not supervising my child while she is in a bathroom with strangers. By the time I'd got myself together and come out, they had left. But my poor innocent little 5yr old Annah was tearing up at the sink where she was dutifully washing her hands. She looked at me in the mirror and said, "Mom that one lady was looking at me mean!". Boy, could it get any worse? I walked all through Winco feeling afraid of turning a corner and running into the woman, who I never saw by the way! But I had seen "her" children and the other woman in the bathroom with them.
So.. if you desire to preserve yourself from such embarassment.. no matter WHAT you think the status of occupation may be for any given stall, like you would teach your children, Knock First!