What does a white dot on an woman's forehead mean?

I've been in the hospital a number of times and every nurse I've had with a white scar on their forehead or the middle of the forehead seems to signify a slow or just plain bad nurse. But in all seriousness, all the nurses seem to be older and a lot seem to have a faded white dot. Maybe because they had their red dot or 'Bindi' removed at a later time or after many years... I'd like to ask, but it's a little out of place I think. In older times it meant a Hindi woman was married, or it was just plain religious culture. A lot seem to have them removed after moving the United States. I guess because of too many questions...