sb got refused in koh phangan for an extension bc a flag. Got extension in samui without problem. So also "being flagged" and the follow up is at the discretion of the officer/office apparently🤷♂️
You should never go on overstay if you can avoid it.
Although it's unlikely with only some days, if you have any interaction with the police (random traffic checkpoint, car with facial recognition camera
or even following a workshop where they check the teacher) while you're even on one day overstay, you can be arrested, taken to deportation jail, and held there until your flight (which has to be a direct one to your home country at your own expenses). Even with one day overstay you'll be banned from Thailand for 5 years when caught.
If you get to the airport then you'll pay a fine (500 per day) and receive an overstay stamp in your passport. An overstay stamp could potentially affect your ability to purchase a visa to another country or enter another country or Thailand in the future.