i have never had a girl "giggle" at me when i say hello and bye. now smiles and hugs and stuff i get all the time but if a girl was giggling at me like that i would ask why. but slip it into a casual conversation like when you are out chilling and just slip it in and say something like "you know what i have noticed about you?" she will say somthing along the lines of "what" or "what did you notice about me" and then you simply have to ask her be like "well everytime i say hello to you you are always giggling, why is that?"

from personal opinion i would assume that they have heard something about you and when they see you they think about it. and then during the middle of the interaction they dont think about it then at the end when they are saying goodbye to you they think about it again. i would not even have a guess to what the thing they know is. but i guess this is all just personal opinion