Congratulations!
You solved the mystery!
"Foster, you've got a population of creatures, somewhere outside your tank," you announce. "They're breathing up all your O2."
Haidee responds:
All right then. Do it by the book. If there are invasive creatures - shut all the airtight doors until we figure out what and where they are.
Foster and the other bionauts close the airtight doors to their bedrooms and storage lockers. And little by little the carbon dioxide level in the main living area of Foster's pod creeps back down to normal.
An investigation reveals that two colonies of termites were hidden in the wooden crates in his storage locker, and some other bugs had gotten into the foodstuffs. All told, over three pounds of insects were hidden in the food supplies.