Ron: Last nite I got to thinking about your question a bit and did a little research.
For p-traps, where the drain pipe comes down from the sink, makes a u-bend then goes into the wall where it ties to the main drain pipe, the answer I gave it correct. However, in many houses with retrofit plumbing, often the drain pipe from the sink will make the u-bend then turn and go directly down into the floor where it ties to the main drain running under the floor somewhere. Often under this 'S' type drain, the vent pipe is some distance from the waste water drain which may cause a big bolus of waste water to suck some or most of the water out of the trap (back at the u-bend). This may allow sewer gasses to come past the trap where there now isn't enough water to block them, and out through the sink. The test for this is to fill the sink with water then pull the plug. According to my plumbing book, if you hear a gurggling and then strong sucking sound at the end of the water flow out, this suggests the trap water is being sucked out. Sometime after this you may detect sewer odors. If this is the case, the only fixs would be to increase the size of the 'U' in the bend, get into the wall and install a dedicated vent pipe up through your roof or refill the trap with a little water after draining out a lot of water.