Id act like nothing happened...Especially if she acts like nothing happened. but I wouldn't try it again.
Well it depends on what type of a person she is and how she views things.
Kissing on the neck is clearly an indication of sexuality if nothing else.
But if I have kissed her on the neck THRICE, (1) I'd know that she sure doesn't have a problem doing it with me (2) If she's a shy and introverted person then I'd know she 'may' not be interested but I'm probably taking advantage of her
That's what it is according to me unless I've misunderstood something in the question