What is a "stop and frisk" search?

In People v. Cogaed,[1] the Supreme Court had an opportunity to exhaustively explain "stop and frisk" searches:
"Stop and frisk" searches (sometimes referred to as Terry searches) are necessary for law enforcement. That is, law enforcers should be given the legal arsenal to prevent the commission of offenses. However, this should be balanced with the need to protect the privacy of citizens in accordance with Article III, Section 2 of the Constitution.The balance lies in the concept of "suspiciousness" present where the police officer finds himself or herself in. This may be undoubtedly based on the experience of the police officer. Experienced police officers have personal experience dealing with criminals and criminal behavior. Hence, they should have the ability to discern - based on facts that they themselves observe - whether an individual is acting in a suspicious manner. Clearly, a basic criterion would be that the police officer, with his or her personal knowledge, must observe the facts leading to the suspicion of an illicit act.

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Normally, "stop and frisk" searches do not give the law enforcer an opportunity to confer with a judge to determine probable cause. In Posadas v. Court of Appeals, one of the earliest cases adopting the "stop and frisk" doctrine in Philippine jurisprudence, this court approximated the suspicious circumstances as probable cause:
The probable cause is that when the petitioner acted suspiciously and attempted to flee with the buri bag there was a probable cause that he was concealing something illegal in the bag and it was the right and duty of the police officers to inspect the same.
For warrantless searches, probable cause was defined as a reasonable ground of suspicion supported by circumstances sufficiently strong in themselves to warrant a cautious man to believe that the person accused is guilty of the offense with which he is charged.

Malacat v. Court of Appeals clarifies the requirement further. It does not have to be probable cause, but it cannot be mere suspicion. It has to be a genuine reason to serve the purposes of the "stop and frisk" exception:
Other notable points of Terry are that while probable cause is not required to conduct a "stop and frisk," it nevertheless holds that mere suspicion or a hunch will not validate a "stop and frisk." A genuine reason must exist, in light of the police officer's experience and surrounding conditions, to warrant the belief that the person detained has weapons concealed about him.
In his dissent for Esquillo v. People, Justice Bersamin reminds us that police officers must not rely on a single suspicious circumstance. There should be "presence of more than one seemingly innocent activity, which, taken together, warranted a reasonable inference of criminal activity." The Constitution prohibits "unreasonable searches and seizures." Certainly, reliance on only one suspicious circumstance or none at all will not result in a reasonable search.[2] (Emphases and underscoring supplied)

[1] See G.R. No. 200334, July 30, 2014.

[2]  See id.; citations omitted.