I’m bilingual since a child but didn’t really use one of the languages much after age 12 to about 30. When I do speak the second language I can fool locals into thinking it’s my first language, but it takes exactly 1 mistake or mispronunciation and they ask “oh are you actually English?”. The bar is that that high for passing native fluency in another language, if you somehow could fake the accent perfectly as well (there is absolutely no way).

Yeah there's definitely a gap between functionally fluent and being mistaken for a native that requires some intentionality and effective study that is unlikely to be crossed accidentally/passively or by focusing on the wrong things/methods.

Some combination of learning the phonetics of the target language, 1000s of hours of comprehensible input, singing to music in the target language, and doing impressions of native speakers are all things that can help.