**Since mercaptan is such an easily traced gas in extremely small amounts, maybe someone was rehearsing just to see how far a lethal gas would go and its likely path.**
My thoughts exactly.
And mercaptan can be used in tiny amounts to make a BIG smell.
I worked twenty years selling propane, which uses the same odorant, ethyl mercaptan used in natural gas. Incredibly smelly, we added one pound per 10,000 gallons by state law. I recall, several years back, south Memphis, trade papers showed the guys in the space suits swarming over a gas plant, only to find the valve on a drum of mercaptan was dripping.
Straight mercaptan will knock a buzzard off a gut wagon, but aside from making you heave if it's too strong, long-term harmless.
This was the first thing that came to my mind. The stuff is sold in one pound cans, could be a merry prankster having fun.