Have you ever heard of a so-called 'mild' Phenylketonuria that can escape diagnosis? I've heard of it mentioned here and there but I'm not sure if it's really w

Have you ever heard of a so-called 'mild' Phenylketonuria that can escape diagnosis? I've heard of it mentioned here and there but I'm not sure if it's really well known (or if it's even proven - I don't know much about it). I know someone who I have a possibly far fetched hypothesis that they might have something on the Phenylketonuria spectrum. That is they are grew up relatively normal but suffering from some severe mental disorders as they age. When they were young they had the characteristic mousey smell that is mentioned in Phenylketonuria.