Alcohol at festivals here is banned to but I have always managed to sneak it in. Not because I'm underage, but because I'm poor and the booze is always so bloody expensive. I can only speak to Australian festivals, yes they are stringent, but no they are not that stringent that they go through every single item in your car like a police raid or something.
Once we simply put all the booze at the very bottom of the pile of all of our camping stuff. We told the officials that if they took everything out could they please put it back because it took us ages to fit everything in the car. They waved us on. I usually put mine at the very bottom of my clothes bag, concealed with clothes or a towel or something. They never find it. They're not going to go through absolutely everything you own.
Some people I know have gone to ridiculous lengths though, like filling up tissue boxes with goon bags and then sealing them up again, stuffing booze down the middle of their rolled-up sleeping bag, pouring booze into coffee thermos...
That being said, if we ARE caught with alcohol on us, it is just confiscated - no fine or anything. Even if there are underage people it's doubtful they'd book us or charge us or anything, they'd just confiscate the booze. So I'm not sure how it works where you are, it's not worth risking it if you can get fined...
Drugs I would never, ever try to bring in because they have sniffer dogs everywhere, and those dogs are amazing.
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