I took some pictures today at a birthday party, A7 and 35mm f2.8 with wide area and face detect set but the face detect kept locking onto a plain white envelope at the elbow of the person sat opening birthday cards... why I just don't know as it looked nothing like a face, just an oblong white envelope. I've never seen this before. Anyway, I switched to single spot and put it on the subjects face. How strange. I suppose the explanation is somewhere in the fact that the envelope was white and it was on a dark chair but I just couldn't see how it could see a face there at all.