His book of etiquette consulted but lacking, the modern gentleman may well wonder what response is appropriate. He has come to the right place.
You have, Dear Reader, perhaps had stuck between your teeth something vegetable; a poppy seed, spinach, whatnot.
Likewise indecent exposure. To rectify one's unintended display, be it buffet or bosom, it is always better to know.
So when a lady has fallen the rest of the way out of her summer top, it is not proper for the gentleman to assume she meant to, whatever the prevailing fashion of the place.
Though he be divided between his own discomfort at raising the topic and the plight, unsightly or otherwise, of the hapless lass, the truly decent gentleman willingly undertakes in all things to subordinate his own comfort in service to the other.
He may not shirk, lest, imagine the tumult, the exposed lady next meet her clergyman or her grandfather.
Neither may he snicker or leer. Leering is to be saved for his wife and snickering for later.
So the gentleman will proceed, delicately, thus:
"Forgive my noticing, Miss, but to save you a felony charge it is my profound discomfiture to inform you that you appear to have misplaced your areola. Not, mind you, that it is lost. Rather, Miss, you see it is very much in evidence. I thought it best you should know. Good day.".
His duty done, he may retire for cocktails.