If it's a sink you don't use very often, the drain water being held in the p-trap (look under the sink for the loop in the drain line) is probably evaporating, allowing any odors from the drain/sewer line to come up through the sink. Ordinarily, the water in the trap will prevent that from happening. Solution: Periodically, run one of the sink's faucets for a few moments to allow the trap to refill with water. Problem solved.
If that's not it..............I'm out of ideas!
Hope that takes care of it for you, though!!
Bill
