when officers discovered that Royer was traveling with an assumed name, plus their suspicions about him being a drug courier (paying cash for one way ticket, mode of checking the two bags, and appearance and conduct); gave the officers reasonable suspicion to believe that a crime was afoot, and could detain him to verify or dispel their suspicions in a manner that did not exceed the limits of an investigative detention. However, what began as a consensual encounter in a public place, escalated into an arrest when Royer was moved, to the police interrogation room (without any safety or security concerns justifying moving him), while the officers kept Royer’s ticket, ID, and luggage (keeping him from being able to leave), while they continued to ask about his actions. The officer’s conduct was more intrusive than necessary to effectuate an investigative detention otherwise authorized by Terry, therefore an unlawful seizure took place.