Police had an arrest warrant for appellant Gorman who was suspected of mail theft using fraudulent mailbox keys. They did not have a search warrant or consent when they entered a thirdparty residence (his girlfriend’s house) to look for him. During the arrest, the officers found the mailbox keys as well as checks issued to other people. Here, the appellate court reversed the denial of appellant’s motion to suppress. Whether Gorman’s Fourth Amendment rights were violated when the police entered without a search warrant or consent depends on whether the police had “reason to believe” that he was present. “Reason to believe” embodies the same standard of reasonableness inherent in probable cause. Here, the District Court equated the “reason to believe” standard with reasonable suspicion instead of probable cause, so reversal and remand were required for further proceedings using the standard clarified in this opinion.