Hiding your gambling activities or lying about it could sometimes be a sign of gambling addiction. Like if you come to think of it , why hide something if it's right or doesn't have something more to it ? One of the biggest issues of hiding gambling activities from the people around you especially family members is when they eventually find out without you breaking the news to them before then. Majority of the they view it as a scenario where the feel they are distrusted by the gambler.
This can cause relationship and emotional issues. Also , as a responsible gambler I don't think you should measure the love you have for your loved ones like your wife with the amount of fun you have when you gamble except you are probably an addict.
I agree it can be a sign of gamgling addiction, but I rather see it as the beginning of a long lasting addiction to gambling. Depending on the context, anyone could start to gamble in secret for a variety of reasons: it could be about religious stigma on gamblers, it could be because it is illegal for people to gamble or bet, it would be even because the person who wishes to gamble does not want the rest of their peers to realize he had money and wagered all of it on a casino.
Though, as losses accumulate and as the person continue to keep the habit a secret, it becomes more likely she/he will never address the problem and open up to their family and friends about it, perpetuating the gambling untill all the assets are gone.
That is one of the reasons I believe talking about losses with our loved ones is important, before they start to accumulate, so we can feel liberated from the guilt and shame which comes with all of it, I assume it helps to move on from behavioral addictions like problem gambling.