If you tune up your detector that high it’s very likely you’ll get false positives. I’ve met Americans with fairly strong “accents” that aren’t necessarily a dialect either, just a different way of speaking. It could also be a mix with parents who are non-native. American English is vast, and very heterogeneous.

If we’re talking about specific parts like a regional dialect then I would agree, those are tricky to acquire later, at least to those undetectable levels. They can be extremely specific.