Can the police stop or detain a person not under arrest

Yes. The police can stop a person, and ask questions, without "arresting" the person. Upon seeing suspicious activity, the police may perform what is called a "Terry Stop," and may temporarily detain people to request that they identify themselves and to question them about the suspicious activity. The scope of a "Terry Stop" is limited to investigation of the specific suspicious activity, and if the police detain people to question them about additional matters, the stop can turn into an "arrest." For their own safety, the police can perform a "weapons frisk" on the outside of a person's clothes (sometimes called "patting down the suspect" during a "Terry Stop." During this frisk, if they feel something that may be a weapon, they may remove it from the suspect for further examination. However, they are not entitled to remove items from person's pockets that do not appear to be weapons, even if they believe that the items are contraband.