I have only approached one diver who had their mask on their forehead to ask if there was a problem. It was not that that aroused my instincts but the fact that she was a student of another instructor. Had lost her buddy and the instructor, was coughing up a little water, and was looking around like "where the hell did they go". I aked her if everything was ok. She said yes and that her mask was actually the problem. It was new and not fitting properly. She just surfaced due to it flooding and was just going to wait until the others came up. I went down and let her instructor know she was ok since she did not signal them but just ascended. Oh, did I mention that the other factor in my taking action was that she was really cute. Not in distress but I would have been happy to "save" her.