On my return recently from a pre-Chinese New Year weekend in Penang, I was questioned by passport control and referred to an immigration officer for further questioning. I was issued a re-entry permit at DMK airport on the Friday, and held on the Sunday night in the same airport. The reason? 4 tourist stamps and suspicion of working, and not looking like a "normal" tourist.
The questions began at passport control, by a relaxed young male employee, who asked me twice, "What do you do in Thai