Which of the following is not one of the six situations in which a warrantless search is valid?

The scenario described in the answer is indeed valid because a consent search requires that the individual giving consent has the authority to do so. In general, consent must come from someone with a legal right to the area or object being searched, which does not always apply to searches conducted by consent for another person, such as an adult family member. This makes it problematic and not classic grounds for a warrantless search, as law enforcement must ensure that the consenting party has the authority to consent to the search of another's belongings.

In contrast, other situations listed are well-established exceptions to the warrant requirement. For example, stop and frisk allows officers to perform a search when they have reasonable suspicion that a person is involved in criminal activity and may be armed. Likewise, an automobile search can occur if there is probable cause to believe that the vehicle is involved in a crime, leveraging the mobility of vehicles as a reasonable basis due to the potential for evidence to be lost quickly. Finally, the plain-view doctrine permits officers to seize evidence of a crime that is visible from a public space, without a warrant, since the individual does not have a reasonable expectation of privacy regarding what is plainly observable.