Well, if the 'happily' were not there, I would elide the comma in an instant. With 'happily', however, and no comma, I don't know for sure whether it refers to 'saying happily' or to his, by serendipity (happily = 'in an unexpectedly lucky way'), making his presence known to her.
Hmm. Would anybody else like to chime in. I think that one would have to completely rework the sentence to get the second meaning that you suggest, Microfdofs.