Exigent circumstances is one of the exceptions to the requirement that a search be authorized by a warrant to be considered reasonable. But exigent circumstances arguments to draw blood without a warrant when a person is suspected of driving under the influence have been often rejected since the U.S. Supreme Court opinion in Missouri v. McNeely in 2013 (determining that the dissipation of alcohol in the blood over time did not create exigent circumstances when it was still reasonable to obtain a warrant). In the recent Tennessee case of State v. Cates, E2014-01322-CCA-R3-CD (Tenn.Crim.App. 9-28-2015), the Court of Criminal Appeals has again reversed a warrantless blood draw.
A warrantless search is presumed unreasonable. It may be reasonable and lawful though if one or more recognized exceptions apply. The prosecution has the burden of proving a recognized exception if the warrantless search is contested. One important exception is consent. People on probation often, as a condition of being on probation, sign a form consenting to the "any time" search of their residence. However, federal and Tennessee law still requires at least reasonable suspicion of criminal activity for one of these "any time" residential searches to occur.
Consent is one of the specific exceptions to the general requirement that law enforcement authorities must have a valid search warrant prior to a constitutional search (where there is also a reasonable expectation of privacy). Under Tennessee law, a person has a reasonable expectation of privacy in the contents of the person's cell phone in his or her possession (an abandoned phone may be a different story). A police officer may only search the contents of the phone if there is a valid search warrant to do so or if one of the recognized exceptions to the warrant requirement is present. In the recent case of State v. Kohlmeyer, M2014-01359-CCA-R3-CD (Tenn.Crim.App. 7-7-2015), the Tennessee Court of Criminal Appeals affirmed a trial court decision that an examination of the defendant's cell phone was valid, due to the defendant having consented to it.