As the person who was audited after retiring from work here, I agree that the chances of it happening to the average pensioner here are very low - or have been up till now. Whether that continues under the new rules remains to be seen. I suspect that it will, but can also see the TRD head office setting targets for tax offices to investigate a small portion of non-filing tax residents in their districts / areas each year. I'd be careful to, at minimum, keep good records over the next couple of years until we see the way things are going.
The main reason I posted my experience, however, was the way that I was informed about the audit. My passport was flagged on the Immigration Bureau computer system, meaning whoever entered my name and number - including departure immigration at every airport and border, got a message saying I was up for a tax audit. As my local immigration office told me, this meant I was prevented from leaving the country until this flag was cleared. (Which eventually I had to arrange myself following the audit, by making a trip to Bangkok to visit the immigration detention centre in Soi Suan Plu).
Therefore, the TRD do have the resources to identify and conduct audits on non Thai citizens, and do have sufficient integration with the Immigration Bureau to firstly identify that people have been in country long enough in a year to be tax residents, and get their passport details (I had obtained a new passport after retiring, so had a new number, but the TRD must have been informed of this by immigration), and then flag that passport in the Immigration computer system. They can, and will prevent you leaving the country until they are satisfied. Anecdotal? Not to me it isn't.
And, while I'm here, I was clearly told during the audit that they use a last in, first out system when assessing remittances from savings. They look at the start and end balances of your overseas account following the remittance. For example, if you have $100k in your account on Jan 1st, you receive a further $50k to that account, then remit $50k to Thailand, the whole $50k is assessable income. You had $100k on Jan 1st, you still have $100k in there following the remittance, therefore that money was "earned" the same year it was remitted.