A bad sequencing control on a boiler. The line itself would be fine and not leaking. But if the control goes bad it could possibly not fire the boiler but allow raw natural gas into the boiler. Boilers use forced draft blowers. If the blower is operating that would take the raw gas on out the boilers exhaust stack usually on the roof. This would likely be a multi story build maybe 5 stories likely much higher. With the area effected it sound like a sizable building.
Natural gas being heavier than air settles to the ground. It would take a building by building search to locate it. A boiler under IIRC 15PSI operation steam pressure doesn't require a boiler operator present which would make immediate detection less likely.
Still doesn't sound like a high enough concentration of mercaptan to me. Mercaptan levels in natgas are extremely small (odor detection levels at the human nose are around 1 ppb). I work in the natgas industry and have experience with mercaptan.
I also know that natural gas is lighter than air at sea level and would not "settle to the ground" (just FYI)