Sink Smell and Slimy P-Trap
Being the highest drain might cause a bit more water to be sucked out of the trap causing some odor to get past the remaining water. If it's a bathroom sink, they're prone to gross buildups due to soap scum, toothpaste, shaving cream and whatever. I have a brush from an aquarium that I run down my bathroom drain and trap regularly and it has helped a lot.