Most people in my country (Non-US) thought that the tax dept will investigate their suspect but the truth is, if you are the suspect, they will ask you to justify and provide proof (legal document, statement etc), to prove yourself innocent. So no matter how hard you hide your income, you still need to find ways to explain, justify and make it looks legal. Unless you never purchase any asset (house, car etc) your entire life.

So the main point is "Don't do something that make you looks suspicious" eg. buy a big house. If one can understand this, then no need to waste energy by trying hard to hide the income because it doesn't helps. It's you who need to provide reasonable answers to the tax dept, not the tax dept to find proof to charge you.