In a divided opinion 5-3 (with No Scalia replacement yet appointed), the United States Supreme Court held in Utah v. Strieff that police may use evidence obtained in an illegal stop, so long as there was sufficient "attenuation" distancing the discovery of the evidence and that stop.
In this case, Mr. Strieff was stopped outside a home which was suspected of drug activitity. Lacking reasonable suspicion (acc to UT Sup. Court), the police stopped and detained Mr. Strieff and began to question him. During the course of the questioning, it was discovered that Mr. Strieff had an arrest warrant outstanding for him. Based on this, he was arrested and searched, where the illegal drug evidence was found.
The Court found that the discovery of the drugs was "attenuated" from the illegal stop because of the intervening knowledge of the arrest warrant.
Dissenting justices Kagan, Sotomayor, and Ginsburg would have affirmed the Utah Supreme Court and ruled that the evidence should have been suppressed because the societal impact of this decision means that officers may go on a fishing expedition, and then justify things afterwards if anything turns up.
Cases like these emphasize that it is important to get a skilled criminal defense attorney who knows Fourth Amendment search and seizure law so that your rights are respected.