The only purpose of day 5 test is scamming tourists and ruining their vacation.
You do a test 3 days before you arrive here and another one in the airport, both come back negative. You do another one in day 5, it comes back positive, and they think you brought the virus with you from your country or airplane!!
If you test positive in day 5. The greater odds are you caught it here in Thailand.
And if you catch it here from someone already roaming freely why on earth only new tourists have to quarantine?
Luckily I came here for long stay so it doesn't bother me that much but in the next room people who came here for 2 weeks with their family and now have to spend the rest of their vacation in quarantine!
We all took the risk to come here knowing this can happen. But most of us(Me included) wishfully think it's probably not gonna happen to us. But really, if you're not prepared for the worst it's better to wait until this nonsense is over. if you have to come during this time my advice to you is to be extremely careful until you get your second test results and avoid bars and crowded places at all cost.