Even though it isn't blocked, the trap may contain bits of discarded food. These bits of food will start to break down producing the 'odours', especially when slightly disturbed by runing water.
If ordinary bleach doesn't clear it, just unscrew the trap, take it outside and blast the **** out of it with a hose pipe.
Also, put a bowl underneath and with an old dish brush, scrub the down pipe from the sink until there's no more gunk left.
Finally, replace the trap, and wash sink with bleach and water.