I personally know a man who has been working here for around a decade. His income in Thailand, even taking into account his personal allowances, means he has to pay tax. However, he has also been receiving an income from overseas, all remitted in the same year as acquired, that has never been declared. He would be in a whole world of pain if audited, please God he wont.
I know some posters will say that the likelihood of that happening is remote, but that is neither here nor there. I am just responding to your valid point however likely or unlikely he gets rumbled.