Ok, so you let the FCC inspector in, he sees your pot plant, and goes to the cops. They get a warrant, search and arrest you. Considering that the warrant was only obtained because of evidence from an unwarranted search, how is this any different than simply allowing evidence from unwarranted searches to be used against you?
Okay, two solutions:
1) Require the FCC inspector obtain a warrant -- this is probably easy enough. If there's detectable interference, the FCC inspector should be in there anyway.
2) Stop growing pot.
Not mutually exclusive, either.
The fourth amendment was never designed to prevent all evidence of crimes from being used against suspects. There's nothing in the fourth amendment that says non-warranted evidence should be ignored, either. That's purely a judicial construct designed to "remedy" bad acts by police. So in your situation, a federal agent on an unrelated matter encounters evidence of a crime, and turns it over to the police. I'm having a hard time mustering outrage that a criminal is arrested when a government agent with authority to be at his location sees evidence of a crime.
Let's amp it up a little, too, since pot stirs arguments about intrinsic criminality. Let's say the FCC agent encounters child abuse in progress. Or a literal smoking gun and dead person. Or a kidnapped person. Or residual evidence of past violence -- blood or a bloody weapon, say -- which leads to a DNA sample identifying a past convicted felon. In each case, the evidence from the unwarranted search leads to a police warrant or testimony, or other evidence offered against a suspect. Are all of those improper? I don't think so. The fourth amendment is not a magic wand to remove evidence of a crime.
Sure, the FCC inspector might be mistaken. That doesn't mean he went in with the intent of violating the person's search and seizure rights. And if the government agent is not trying -- intending -- to violate those rights, it makes no sense to exclude the evidence. That's why there's fourth amendment exceptions, like public safety, and genuine inadvertent error. Throwing out evidence when the police did nothing wrong doesn't serve anyone's interests but the criminals. And catching criminals is in the public's interest.
And let's get real: the local cops don't have FCC inspectors in their hip pocket. If the FCC is there, it's because the FCC should be there. In which case, requiring them to get a warrant won't stop them; they'll have one.
This whole exercise seems like a red herring to me.