I've gone through the Massachusetts Health application process(the model for the ACA). To qualify for subsidised healthcare, your current income cannot be more than 300% of the poverty line. If you qualify for subsidized healthcare, you are told that you must notify Mass Health if your financial situation changes.
So, if you applied for Mass Health because you lost your job, you can't stay on Mass Health when you find another one.
The lovely Mass Health bureaucrat made it clear that I was susceptible to an audit and fraud charges if I didn't report an "income event" (I believe that's what she called it) in a timely matter.
Her explanation might shed light on the phrase "audit activities". But I haven't actually read the Obamacare legislation, so I can't say for certain.