Boiler feed lines?
Why not a malfunctioning NatGas odorant plant? Enough mercaptan there to stink up the entire metro area.
Does anyone know if these are underground in NYC?
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.