The sinks have traps just like at home. After sitting for an extended period of time, the water in the bottom of the "U" evaporates and will let odor escape from the gray water tank. The solution is to simply run some water in the sink to re-create the water seal. Note, this can occur at home as well if a sink, tub or shower isn't used for a while. In that case, however, you'll get a definite sewer odor.
As blueridge-fl indicated, a gray water tank that's had food scrapings dumped into it can smell. Otherwise, with all the dishwashing and shower soap that goes into them, they usually don't smell too bad if at all. We use a stainless steel screen drain insert to avoid getting solids into the gray tank:
https://www.amazon.com/Discovery-Sta.../dp/B0017HRZZ8
You should also try not to dump liquid food waste into the gray tank, better to use the toilet so it dumps into the black water tank since you'll be cleaning it out more often.