Welcome, this is a great place with great people who are in the same place as you, I hope all the information and comments help.
My AS was a secret drinker, which arose after the first intervention more than 10 years ago. She would stash her big bottles of vodka in a few places and then transfer to water bottles, walking around like nobody could tell or smell she was drinking. Before Al-Anon and SR and not realizing I could control it, we would find water bottles everywhere in various volumes, everyone smelled of vodka.
I think the closet drinker arrises from not wanting to admit there is a problem, not wanting to give up their addiction, thinking they can control it. They even think vodka doesn't smell, adding to the secrecy. The disease needs to be fed by alcohol and if you hide it, they think nobody knows and they can continue to drink.
Of course, any time you confront them about it, there will be excuses, apologies, and more coverup. They may even go a day or two seemingly sober, then back to the secret drinking.
My thoughts and prayers are with you and your family.