Service Call Mystery. Calling all Big Brain HVAC techs

I have this interesting service call Dilemma. I have a weird/slightly complicated scenario.

Every 3 months, the customer smells a “gas smell”. Gas company comes out each and every time, and finds no leaks or traces of gas.

I found the MUA until that pulls in 100% supply to the building. I checked surrounding units for potential misfiring. My thought was that a misfire from the surrounding units, or a gas burp would be absorbed into the MUA.

At this point, I’m attempting to replicate the smell that they had coming from the MUA unit, or from its own gas Burp. It is a complex trane unit that would absolutely crawl to attempt to get it high enough to open feed on a modulating gas regulator.

Any smart ideas on how to absolutely prove a gas purp that naturally only happens every 3 months.