This is a stretch, especially weeks after the disappearance, but maybe Beth smelled Natalee's perfume there? My daughters all used distinctive scents, and when walking into a room, I could tell which one had passed through. (Certainly not after days, though). One always wore a musk; another, lavender; the third one, her mother's Chanel.
Maybe a stretch, maybe not. What if something of Natalie's was still there...there could be a scent on something. Plus a Mother can sometimes sense things about her children that NO ONE ELSE could possibly have picked up. To a kid, that can be an advantage or disadvantage but that's the natural beauty of a Mother's ability to protect her young, IMHO!
Maybe a stretch but who knows....