It could be a different maxi dress. If you look closely at the undated picture the FBI used for her 'missing' poster, that dress has spaghetti straps--and on a pic provided by RF, they criss-cross or X in the back.
If you look at the shoots from hours before she went missing, that dress has wider staps that do NOT crisscross in the back, they just go straight down. She may have packed two very similar looking maxi dresses and wore one Sunday and the other Monday.
Dress one:
http://www.nydailynews.com/topics/Robyn+Gardner/photos
http://www.radaronline.com/photos/image/140662
Dress two:
http://articles.nydailynews.com/201...n-authorities-dutch-caribbean-island-giordano
That could be.