What does Doon discover in tunnel 351?

In Jeanne DuPrau's sci-fi novel The City of Ember, twelve-year-old characters Lina and Doon are citizens of a post-apocalyptic society, living in an underground city called Ember. Together, the two work to figure out important instructions left behind by the city's founders, as well as why Ember's electrical system is failing and food supplies are disappearing.
In chapter 12, which is titled "A Dreadful Discovery," Doon returns to the locked door in tunnel 351 only to discover there is now a key sitting in the lock. When he opens up the door, he finds a "brightly lit room" that is filled to the brim with "mounds of cans, heaps of clothes, rows of jars and bottles, stacks of light-bulb packages." These are all valuable, necessary resources that are being kept from the people of Ember. Doon also sees "a great blob of a person" asleep in a chair, with a plate smeared with food in front of him. While talking with Lina later on that day, Doon tells her that the person he saw is the mayor of Ember, who has apparently been stealing and hoarding these supplies for himself with help from Looper.