After learning that the person calling themselves Leet is really Jimmy the Thumbs in Beyond a Steel Sky, his final words are that the real Leet is in the Hellfire Club. If you go to a monopod, you can’t travel to the location. It likely means the furnace that is in the other building where you filled Jimmy’s moonshine bottle. You need to turn the furnace off to investigate what’s inside of it.
There are no valves inside of the building that turns the furnace off. You need to proceed back outside of the building and go around it until you find some stairs. There’s a gas control valve on the side of the building that controls how much goes into the furnace. However, when you try to interact with the valve, it’s rusted, and won’t turn.
Go to the ladder on the side of the large tank and proceed to the wire behind the sign. It’s holding up a large anchor. Approach it, and use the cleaver that Jimmy the Thumbs had planned to use on you to sever the anchor and cause the valve to leak gas out. Proceed to where the anchor landed, but before doing so, approach the corroded light switch to turn off the lights below the sign. There should be a broken lamp next to the flowing gas. Place the plastic unicorn from Jimmy the Thumb’s desk inside it because it contains C4. Return to the light switch and flip it on to blow up above the building.
The furnace should no longer contain flames inside of it. You can now freely explore it to learn more about what happened to the real Leet.