When is a 'stop and frisk' search considered valid?

Dear PAO,

X was arrested for allegedly having in his possession illegal drugs. He and his cousin were just two of the few passengers on board a bus on the night when the arrest happened. According to the report that was shown to the mother of X, the police received a tip that a male passenger of a bus was carrying drugs which are to be sold in the province. So, the police allegedly hailed the bus and frisked the passengers, and thereafter found the drugs inside the bag of X. But according to X and his cousin, X was not carrying any drugs and the supposed drugs found on X's bag are merely planted evidence. The police also did not introduce themselves, nor did they present a warrant, before searching the passengers, and there was nothing out of the ordinary inside the bus that would suggest that a crime is being committed or will be committed. So, all the passengers that night were really confused and shocked. Is it really possible for the police to just warrantlessly frisk someone, even if that person was not doing anything out of the ordinary?