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Let's say I am accused of stealing a refrigerator and the police believe that you are not involved, but the cassette tape in your answering machine may hold indirect evidence of this theft.
You are the "communications provider" in this metaphor. Your house is being searched for evidence against me but, you still have fourth amendment rights.
You don't "lose" Fourth Amendment protections against searches only because you are a suspect. The police have a right, with a properly executed warrant, to search my house based on probable cause that my house might have evidence of a crime you committed (even if I don't know anything about it). This is a reasonable search, and the Fourth only prohibits unreasonable searches (a right I still possess). Both you and I are nominally innocent people at the time. And may still be. That doesn't prevent the warrant from issuing.
Cheers,