I think it's a bit strange when the family doesn't know, because a gambler will definitely show unusual behavior, such as their money running out quickly or other things related to money. For me personally, from there, questions started to arise, why did his money run out so quickly, for example, and that could be the start of following up on what he was actually doing. I don't think it's possible for a gambling addict not to exhibit unusual habits, especially in financial terms.
But then again, Asian Families are tight-knit, and if you don't have any probable cause, most of the time your relatives will dismiss any allegations against you and may even defend your name, even if it's a transgression you committed against them. I could see the parents of the kid being suspicious and doubtful of the man's actions especially during the days of the kidnapping, but needed verifiable evidence that the kid is indeed within his care (for a lack of a better word). Could also see this guy concealing his addiction from the people around him or the other way around which may have lead people to suspect him as the kidnapper and apprehend him in the process.
Basically, it's all ahout culture and tradition.