nited States v. Dionisio, 410 U.S. 1, 11-12 (1973) (quoting Hale v. Henkel, 201 U.S. 43, 76 (1906), abrogated in part on other grounds by Murphy v. Waterfront Comm'n of New York Harbor, 378 U.S. 52, 68 (1964)). The government engages in a "search" for Fourth Amendment purposes when its conduct encroaches on an individual's legitim