I am not sure what the difference is between the yellow and red markers on the gas leak map, but I called NStar to check on a gas smell at my place in February or March, and they came out within a day and measured. He even measured over by the neighbors. Everything was within safety limits, and he explained it was probably the exhaust from my high-efficiency boiler (the smell was intermittent, and there is none now that its warmer out). Hope this helps. Thanks for the work you are doing!