You may be right... Some of the pages I found from Googling claimed that the "plain view" exemption also applies to hearing, smell, etc., but I'm having trouble finding an actual citation. The closest thing I can find is Kyllo v. US, where the Supremes said that you need a warrant to use thermal imaging to look for the heat of marijuana-growing lamps, even if you never step onto the suspect's property to use the imager.