Hello!
I have a fifteen year old, too. I've never caught her sneaking out, but I've caught her drinking. I found her making herself a cocktail in our dining room one night. She started crying and admitted she'd done it a couple times before.
Ultimately, we decided not to punish her. We talked to her about her decisions and we took all of the alcohol out of the house. We didn't yell at her or ground her or take away her phone.
We decided there's no point in punishing her for something my husband and I were both doing at that age. It's not at all uncommon for kids that age to drink. It's silly to get upset and act alarmed. It would make us seem scary and out of touch and, ultimately, we'd end up LOSING authority.
Like you, we decided to focus on being safe. We told her that, while we don't approve of her drinking at this age, and we hope she will not drink, and we're not going to make it easy for her to drink, or be the kind of parents who allow it, we understand that it's something that is around and we will not punish her for it. Our hope is that she learns to drink responsibly and safely. That means never getting behind the wheel or getting in a car with someone who has been drinking. It means making sure she drinks slowly. Pace yourself and don't do a bunch of shots in a row or use beer bongs, because if you get blackout drunk you're putting yourself in a risky position. We told her we will always come and pick her up if she's in a bad place. Most importantly, we won't get upset or judge her. We also talked to her about alcoholism. We told her about people we know in our family and outside of it who are sober and have gotten a lot out of 12 Step programs. If she ever ends up in a place where she thinks she might have a problem, there's a way out and there's no shame in getting help.
It went really well and I'm so happy!
After that talk, she drank once more at a New Year's Eve party. We have a big family and there was a big party at her grandma's house. All of the adults were having wine and she was sneaking it. She admitted it to me later. I kept my word and didn't get upset at all. I just rolled my eyes like it was silly.
Since then, she has not had any alcohol. She tells me that she has friends who party and she feels a little babyish for not being like them. They've all had their first kisses and she's never kissed a boy. When I asked her why she doesn't do that stuff, she just shrugged and said she's not interested in it. The great thing is that she came to that place on her own. By focusing on good decisions and safety and not creating some culture of fear around our house, she ended up making good decisions for herself.
She's a straight A student in honors classes at a very competitive school. She's on the debate team. She's an editor of the school newspaper, does mock trial, plays piano in a band, as well as a classical ensemble group. We are close and she shares everything with me. We have a lot of laughs and we have dinner together every night. Her and her younger sister are really tight.
I know that my handling of the situation isn't conventional. Lots of parents would probably say I was too permissive. I disagree. My daughter is awesome and she's safe and we're close and she trusts my opinion and listens to me about a lot of things because she knows I don't have silly rules. She knows when I tell her she shouldn't do something, it's because I know what I'm talking about and she listens.
If I were you, I'd have a really frank talk with your daughter. I'd reassure her up and down that NO MATTER WHAT she told me there would never a bad consequence for telling me the truth. I'd ask her what was going on in her social group. I'd ask her WHY she snuck out and why she drank. Curiosity? Peer pressure? A little of both? Did she like it when she did it? What happened that night? Where did they go? What did they do? I'd make a plan for this sort of thing in the future. Maybe, in the future, she can tell you where she's going and reassure you that she's getting a safe ride and with good people. You can make a deal that she can go to the party or whatever, but she has to text you every hour and keep her Life 360 on and, if there's drinking, she has to let you pick her up. And, if she comes home, and there's alcohol on her breath and you didn't pick her up or she didn't text... if the deal gets broken... then, there's a form of punishment. That way, the punishment is for LYING and RISKING HER SAFETY, it isn't for doing something that's normal (drinking) or for telling the truth.
My daughter told me that she loved the way it made her feel relaxed and free. I said something along the lines of, "That's why most people drink. Most people use it to relax. It's an easy way to unwind and escape stress and anxiety. The problem is, some people never learn to relax any other way." My daughter is a really type A person. I encouraged her to exercise, to try a yoga class, to journal, to develop a relationship with God. Belief in a higher power has helped me immensely with stress. I wanted her to find lots of ways to relax and have fun.
My younger daughter, who is 13, was invited to a sleepover at a girl's house. Without going into all the details, I found out the girl vapes. Of course, it made me really nervous!! I was NOT happy that she'd chosen to be this girl's friend, BUT the girl was really popular and my daughter wanted to hang out with the popular group. Which is so real! I knew I was never going to talk her out of wanting to be popular. Instead, I said that I trusted her not to vape. I said that I was going to be really nervous about this sleepover, so I asked her to please text me often that night and let me know it's all going well. It went well and she texted me frequently and she did not try vaping. In fact, after a few more hangs with that girl, she naturally moved away from the friendship. The girl started getting in lots of trouble and my daughter was uncomfortable. It also laid the groundwork for more trust between us. She knows she can tell me what's going on and I'll trust her to do the right thing and help her.
We also use a safety phrase in our house. We came up with a question that is a code phrase. If they ask me, "Is there any pineapple in the fridge?" It's a code phrase for me to come get them. I say that there's an emergency at home and I need them to come home. That way, they don't have to make up an excuse and I can get them out of whatever weirdness is happening.
I've told both my daughters multiple times that I can't be standing over their shoulder and keeping them safe from the world their whole lives. They have to make good decisions. They have to love themselves and care for themselves and be smart.
I feel like I'm going on and on, but you see what I'm saying. An open, honest relationship is way more valuable than punishing a kid. It's much safer for the kid and they end up making better decisions for themselves because it's of their own volition.
Your instincts about focusing on safety are really good and I've found that approach worked out really well in my house.