There's a lot of play in the joints when courts review Miranda custody deteminations as to what exactly are the facts below as compared to legal determinations. The former are reviewed only for clear error; the latter are viewed de novo. For an interesting example of the tension, check out the divided decision in United States v. Bassignani today. Judge O'Scannlain concludes that the suspect was not in custody. Judge Bea, dissenting, concludes that the key factual findings require the court to affirm the District Court that the defendant was in custody. Interesting case.