He can actually use "hiding" because gambling is kinda stereotypical in many climes and you can blame the society but the irresponsibility of many gamblers. Many are too idiotic with gambling and have wasted their fortuned to it, so the wide scope of view is why some people might actually be hiding. Also, some irresponsible gamblers themselves who are already addicted to it could be hiding after much advice from people despite it affecting their lives. These are the two most important reasons why gamblers could hide, while the third could be for personal reasons. Some people are just introverts and private personalities, they hate people knowing whatever they are doing even if they are the most responsible person on the planet earth. To them, they will always prefer to play in their closet, but for me, we can't particularly call this hiding, it's just their preference.
You are right, the society one find themselves in matters, if those guys are from society where people see gamblers as irresponsible people then I don't think they'll want to do that openly, security too matters since a huge money is involved like the OP said, maybe they're hiding it from attraction of the public or maybe they're gambling secretly to avoid distractions from the public or so.
Religion has a role in that too, religious leader who gambles would not want his follow brethren to notice his gambling habits especially when his Religion doesn't tolerate gambling, also a gambling who comes from a family were gambling is prohibited would not like to be caught gambling in public, sometimes it could also be peoples personal decisions too, those are the things i think makes people gamble secretly.