I don't think it's unreasonable for them to expense meals. Most jobs that take one away from home have some form of expense or per diem.
I'm not well educated on the finer points of a Senators income package, but if they have an expense system rather than a daily per diem than damn straight they need to account for every cent that is spent - and the expense must be reasonable.
This investigation is looking into such small details because the system as a whole has been abused. I won't stray into the politics of whether we should have a Senate or not, but all politicians - elected or otherwise - must understand that everything they do is a matter of public record, including what they had for breakfast.
I get a call from the accounting department if there is something wrong with my expenses, so why shouldn't our leaders? If they want to grumble about the "intrusiveness" of this audit, then they should pack their bags and go back home...their real one that is, not the one they expensed out...