The case in question centered around Antoine Jones, a Washington, D.C. nightclub owner and suspected drug dealer. Police installed a GPS device on Jones' wife's Jeep Grand Cherokee, which eventually led them to a substantial amount of cocaine. Jones was then sentenced to life in prison.
But Justice Antonin Scalia wrote that installing the device constitutes a search under the Fourth Amendment. Specifically, Justice Scalia said, "The government physically occupied private property for the purpose of obtaining information."
In the Jones case, law enforcement agents had obtained the proper warrants to attach the device, but didn't manage to attach it within the 10 days specified in the warrant. The FBI accumulated 2,000 pages of data over four weeks by tracking the vehicle.