You need to give her more freedom. She will only get more sneaky and do things behind your back if she feels that your restricting her. Obviously you need to keep her safe but at 15 it needs to be more of a two-way street on boundaries, so you need to give her trust and pull back when she needs more guidance. If you completely ban drinking she'll do it anyways, if you have a sensible conversation about the risks and benefits and say she can only have x amount it's more likely she'll listen because it's fair. There's nothing wrong with a little bit of drinking at 15 (in fact having some at dinner or showing her moderation can be good). Many people I know at 15 were drinking a heavy amount because it was so restricted and doing drugs and many other things. Another thing would be letting her have sleepovers with people you know and if she does well, then giving her more freedom. If you want her to come to you when things happen then you need to trust her. You should want her to think, "Oh shit this happened, I need to go to Mum to see what I should do because she won't judge me but she'll be understanding". Not how do I make sure mum doesn't find out, it will reduce a lot of pain in the future.
She's 15. If you cannot have reasonable and educational conversations to explain your reasoning then she won't care. Like with sex - the number of parents who have one conversation that goes like don't get pregnant and about nothing else but say they've done the talk is baffling. So then so many young people don't know where to get info and find misinformation or dangerous content, when there's not just 1 sex talk but dozens of things like porn and sexting and different types of sex and different types of contraception and what makes up consent, contraception and pleasure. The parents who have these regular chats treat their children with respect and like adults by giving them privacy, not going to see where she is on a sleepover (which I would say is an invasion of privacy, that you should only do if you think something quite dangerous is happening). You should have called first and see what she said and texted, then maybe contact the parent whose house she's at. It would have to be a major thing for you to go find her, trust her first by calling and asking. Then she feels less controlled and more respected and then she's more likely to listen and come to you, rather than just be sneaky. I can see the signs of this based on the fact she wants to turn of her location, likely cause she feels you are always watching and controlling her and see wants some freedom. I'd have a good conversation with her (regular check-ins) as she gets older about what she also thinks is reasonable and fair and then she feels that she's not being controlled but that she's involved. You have to let her do teenage things, whether that be going to a concert (with people you trust, she has to be back by x) if that goes well then you try something else and so on and so on. Even if she messes up, you don't completely stop her from doing it again, you talk about it, the peer pressure, her reasons, you educate her on why x is a bad choice. Then you let her try again etc, it's better she does this in the next few years whilst you can still help and guide her then you don't let her and when she's goes to uni or moves out she goes wild. Or even starts to go wild now, and in school and do things behind your back and perhaps get into dangerous situations.
Also have these conversations and boundaries, it's only fair she knows them and you need to let go quite a bit. That's what good parenting is, giving them independence and freedom (with the caveat that you are there and can guide them and give more help when needed) otherwise most of the time they pull away from you and that is not what you want. It's hard for parents to treat their children more like adults and also to let them have more independence because you love them and want them to be safe, but in my opinion, if done well in the long-run it will make for a good trusting relationship.