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Unfortunately there is nothing wrong with what the police did here. The homeowner gave voluntary permission for the search. The fact that police were not looking for marijuana plants does not mean they have to ignore contraband when they see it in a place where they are lawfully searching. There is something called the "plain view doctrine" that was established decades ago by the Supreme Court. It basically says that a seizure of contraband without a warrant is legal if:
Here it sounds like all of those elements are met. 1 and 2 are met by the homeowner's consent, and I am sure the police were able to recognize a marijuana plant to meet no. 3. Typically the officers would have to have a warrant to do such a search, but the homeowner's consent as well as the plain view doctrine are exceptions to the warrant requirement so they are considered legitimate searches. Sorry I can't give you better news.