I'm not sure the name "hidden secret" canyon applies too well these days, as there are signs pointing to where you can do it and it's the first place popping up when you search for "hidden secret" in Bali on

With Bali being such a popular travel destination it didn't even occur to me that we could find a secret waterfall, but we did.   How we found it My friend had the number to a local driver that she uses when in Bali