Common Law Searches
To determine whether a search is authorized by the common law, a court must determine:
(1) whether the police conduct in issue falls within the general scope of any duty imposed on the officer by any statute or common law, and
(2) whether, in the circumstances, the police conduct involved a justifiable use of the powers associated with the engaged duty.
R. v. Waterfield,  1 Q.B. 164;
R. v. Godoy,  1 S.C.R. 311, at para. 12.
Searches incident to investigative detention
One line of jurisprudence concerning when a search is justifiable arises from searches conducted pursuant to an investigative detention. In R. v. Mann, 2004 SCC 52,  3 S.C.R. 59, the Supreme Court established the following four requirements.
1. The police officer must have reasonable grounds to suspect in all the circumstances that the individual is connected to a particular crime and that such a detention is necessary: Mann, at para. 45.
2. The police officer is entitled to search the individual detained for a weapon where the officer has a reasonable belief that his safety “or the safety of others…is at risk”: Mann, at paras. 40, 43, 45. The Supreme Court cited with approval the decision in Minnesota v. Dickerson (1993), 508 U.S. 366, at p. 373-74, which describes the objective of a valid protective search as extending to “bystanders”. The decision to search cannot be premised on hunches, mere intuition, or a vague or non-existent concern for safety, rather, the officer, “is required to act on reasonable and specific inferences drawn from the known facts of the situation”. The search must also be confined in scope to an intrusion reasonably designed to locate weapons: Mann, at paras. 40-41.
3. The search must be conducted in a reasonable manner: Mann, at paras. 45.
4. The investigative detention should be brief and the individual detained is not obliged to answer questions: Mann, at para. 45. Questions during the detention may, depending on the circumstances, amount to a search and seizure of information.
R. v. Lee, 2017 ONCA 654.
A search incidental to an investigative detention is defined and limited by the immediate concerns of officer safety. This reflects an important difference between the narrowly focused and strictly limited protective search that may accompany an investigative detention, and the broader power to search consequent to a lawful arrest. It is necessary to maintain that distinction and to confine the scope of a search incidental to an investigative detention within strict limits.
See R. v. Plummer, 2011 ONCA 350, 272 C.C.C. (3d) 172, at para. 76.
[In R. v. Plummer, 2011 ONCA 350, 272 C.C.C. (3d) 172 a police officer conducted a pat-down search and then conducted a more extensive search of the suspect’s vehicle for weapons after the pat-down revealed the suspect was wearing a bulletproof vest. The Court of Appeal for Ontario held that search was a valid use of the power to search incidental to an investigative detention.]