I'm not in Thailand but I've spent a lot of time there, although I've never detected over there. If I was back over there with a detector (and time!) I would try to hunt around old ruins. Obviously I wouldn't go hunting in the protected areas, but there are tons of spots where the only indicators are a crumbling old stupa or the base of an old wall. You are in the heart of Khmer country.
That being said, use common sense. If you are Thai you can get away with a lot more than if you are a farang. Thais tend to feel like if another Thai is taking something it is ok, but if a farang is doing it he might be stealing their heritage. I dealt with this a little in my research on the plants of Thailand if I was collecting plants (legally!) without a Thai escort. If someone hassles you just move on.