I manage a highrise building and the smell you are referring to used to occur all the time and was hard to locate. Finally we realized that rarely used tubs, sinks, drains etc can let that sewer gas smell come up if we dont keep the traps wet. If you dont use that 1/2 bath often, turn on both the hot and cold water and run water for 10-15 seconds each week. Even the water standing in the pipe to the faucet can get stagnant and needs to be cleared out.