I will specify that people born into bilingual families do not count. It must be someone who started learning the language after childhood (where one's susceptibility to language is virtually cheating).
Can someone who learns Japanese, say, with English as their native language be completely (not nearly) natural in both mannerisms, speech, etc., to be perfectly indistinguishable from a native speaker's POV? Meaning, if a Japanese person were to converse with this person on the phone using all colloquialisms and idioms, one would never get the faintest feeling that something was "off" about them?
I'm curious if any major studies have been conducted which examine something like this. It is frequently found that people who have dedicated their life to intensive study and immersion still find themselves occasionally lost in a conversation in that language.