Using his analogy it seems more like you sent an invitation which the fbi returned to sender with a gps device. The FBI didn't search the unsent letters, they returned one that sent itself.
Where there not a similar case where then police bugged a car with a gps device, without a warrant, and the supreme court decided unanimously that police agencies must obtain search warrants before they can install GPS tracking devices on the vehicles of suspects?
It also seems to miss the point. If a person has a hidden identity and a stalker breakers into the post officer in order to obtain the protected address, is the stalker committing a crime towards that person or can they just claim that addresses "is public" because the sender "voluntarily shared the information". The fact that the sender has protected identity should give a strong hint towards their intentions, even if the knowingly used the postal service.
If the FBI used a sybil attack and only passively gleaned the information as the "letter" came past, rather than do a breaking and entering, would it change anything? We could still distill this legal issue as intentional protected information being knowingly intercepted without a warrant. It would be a interesting case, but first the court need to decide if planting malware on citizens computers need a warrant or not.
