Consensual encounters - Voluntary encounters - Knock and talk
See Florida v. Bostick, 501 U.S. 429 (1991) There is no doubt that, if this same encounter had taken place before Bostick boarded the bus or in the lobby of the bus terminal, it would not rise to the level of a seizure. The Court has dealt with similar encounters in airports, and has found them to be "the sort of consensual encounter that implicat[e] no Fourth Amendment interest." Florida v. Rodriguez, 469 U.S. 1, 5-6 (1984). We have stated that even when officers have no basis for suspecting a particular individual, they may generally ask questions of that individual, see INS v. Delgado, 466 U.S. 210, 216 (1984); Rodriguez, supra, 469 U.S. at 5-6; ask to examine the individual's identification, see Delgado, supra, 466 U.S. at 216; Royer, supra, 460 U.S. at 501 (plurality opinion); United States v. Mendenhall, 446 U.S. 544, 557-558 (1980); and request consent to search his or her luggage, see Royer, supra, 460 U.S. at 501 (plurality opinion) -- as long as the police do not convey a message that compliance with their requests is required.
Also, see United States v. Watson, 423 U.S. 411,423-24 (1976). No warrant is required to arrest someone in a public place, to fully search them incident to that arrest, including any containers they might be carrying, or to "stop and frisk" them.