What is the test for a motion to suppress evidence?

An appellate court's review of a motion to suppress evidence is also governed by well-settled principles. The trial court's factual findings relating to the challenged search or seizure, "whether express or implied, must be upheld if they are supported by substantial evidence." (People v. Lawler (1973) 9 Cal.3d 156, 160, 107 Cal.Rptr. 13, 507 P.2d 621.) " 'The trial court also has the duty to determine whether, on the facts found, the search was unreasonable within the meaning of the Constitution.' (Ibid.) Because 'that issue is a question of law,' the appellate court is not bound by the substantial evidence standard in reviewing the trial court's decision thereon. Rather, ... in such review it is 'the ultimate responsibility of the appellate court to measure the facts, as found by the trier, against the constitutional standard of reasonableness.' (Ibid.) On that issue, in short, the appellate court exercises its independent judgment." (People v. Leyba (1981) 29 Cal.3d 591, 597, 174 Cal.Rptr. 867, 629 P.2d 961, fn. omitted, quoting People v. Lawler, supra, 9 Cal.3d at p. 160, 107 Cal.Rptr. 13, 507 P.2d 621.)