The reason why 'Sea' is a hidden track:
Q: And if fans are so lucky to own the physical album, they’ll hear two hidden tracks at the very. Why keep them secretive?
Namjoon: I think they’re hidden because you have to be a real fan of BTS to understand them. Otherwise, you won’t. Otherwise, you’d like be, “Why are they feeling so confused about things? They’re good?!? They’re No. 1 somewhere, they have so much stuff, why are they worried?” People always talk about that. But if you are a true fan of BTS and you buy the album and you listen to the hidden track – if you are an Army and we spent time together from 2013, 2014 – they could understand. It’s kind of more special, more closer, to our true hearts.