Answer: The police can certainly rely on their senses (sight, smell, hearing, touch, and even taste) when deciding whether they have enough credible information to justify seizing an item and searching it. But the Fourth Amendment generally requires law enforcement to obtain a warrant before they actually seize the item. However, when officers see evidence of a crime or contraband “in plain sight,” they may seize it without a warrant, as long as they are standing in a place where they have a right to be. The theory is that there hasn’t been a search at all when a person leaves items in the open, for anyone to see.
So the question is really whether the officers obtained a warrant before opening your package, and whether they had enough information to justify the judge’s issuance of the warrant. If they did not get a warrant, can they argue that “plain smell” alone is enough to validate their search?
If the officers stopped by the courthouse on the way back to the stationhouse, they might have given a judge a sworn affidavit, explaining their thorough training in the detection of marijuana, their experience doing so, and the nature of the smells that emanated from your package. A judge may well have had enough credible information to issue the warrant, especially if the officers could point to corroborating evidence, such as the presence of residue on the outside of the package.
Not all states will elevate “plain smell” to the status of “plain view.” Your state may not consider the smell of marijuana to be so clear and known to everyone that it will, by itself, justify a warrantless search. In California, plain smell alone won’t support a search without a warrant. (Robey v. Superior Court, 2011 DJDAR 15551.)