Whatever process is causing the presence of methane can apparently produce hundreds of thousands of tonnes of the gas in a relatively short timeframe.
Considering that spacecraft have only landed on Mars in the last few decades, a contaminant microbe would have to truly thrive in an extremely inhospitable Martian environment, survive spaceflight and decontamination, yet also be undetectable in the places on Earth where the spacecraft was constructed.
Yes, it's possible, but it seems so beyond the realm of probability...
Considering that spacecraft have only landed on Mars in the last few decades, a contaminant microbe would have to truly thrive in an extremely inhospitable Martian environment, survive spaceflight and decontamination, yet also be undetectable in the places on Earth where the spacecraft was constructed.
Yes, it's possible, but it seems so beyond the realm of probability...