Home is approximately 6 years old, slab floor, two full baths connected to a septic system. Whenever, throughout the day, a sink drain has not been used in a few hours and you open a faucet to wash or get a drink of water, as the water starts to run down the drain if you are washing your face and have your face over the sink you get a hint of sewer gas that is emitted up through the drain pipe. This happens in each of the two bathrooms and in the utility sink in the laundry room and since the kitchen sink is used the most often, however this occurs rarely but at times a sewer odor is noticed here also. The venting for each of the p-traps is through the flat one story roof. This has occurred since I purchased the residence and the contractor and plumber and the inspection folks all indicate that the system was assembled according to code and this shouldn't occur - but it does. Any suggestions as to how to eliminate the small blast of odor?

Your vent may be installed correctly but it sounds to me it is blocked. A block vent can easily cause the traps to siphon when water drains through them. Witout the proper level of water in the trap your have a direct pipeway to your septic tank. Rod the vent from the roof with enough snake to reach roof to slab plus 25' more.