What is the test for establishing cause of action where a party has been spied on by a third party?

To begin with, "clear indication" that the intrusion will supply substantial probative evidence is a sine qua non (Cupp v. Murphy, 412 U.S. 291, 295, 93 S.Ct. 2000, 2003, 36 L.Ed.2d 900, supra; Schmerber v. California, 384 U.S. 757, 770, 86 S.Ct. 1826, 1835, 16 L.Ed.2d 908, supra ). This requirement, by insuring that the evidence expected to be found is of importance, guards against a "fishing expedition". Needless to say, most often facts which would establish probable cause will also tend to establish the high degree of relevance the nontestimonial evidence sought would have (Schmerber v. California, 384 U.S. supra, at p. 770, 86 S.Ct. at p. 1835).