I've probably listened to Mystery Night more times than both TokiSora/Password is 0 combined. MM. definitely got better production but as a song I just really really really enjoy Mystery Night.
Tsunku says: (Mystery Night) Going by the outfits, I'm sure it looks very sexy, but the song is about the loneliness girls feel. The violin reflects this very well.
And I agree, completely. The song does seem really lonely.