Lloyd I "think" everyone on the planet knows what an extension is... No need to be ignorant. Let me rephrase, I have never heard of an immigration official asking for proof of employment when leaving a country including Thailand, even on an extension
I have never heard of an immigration official asking for a work permit anywhere when leaving a country. It seems very odd. The Visa is what gets you from point A to B, not your work permit. If your Visa is within date range, you should be able to come and go in theory
When I was there, that's what they told me. If I was a patient or getting treatment I could extend. I hope folks don't abuse a great service to their patients.