If I'm a cop, standing on a public sidewalk, looking into the window of 123 Main Street and see a plant that I know is marijuana, can you enter 123 Main Street under plain view?
No - you have no lawful right of access. You must have a warrant or an exception to the warrant requirement to actually enter 123 Main Street. Use what you've got to get an arrest warrant or a search warrant. Maybe there is an exception to the warrant requirement that might let you inside - if you knock on the door you can get consent. If somebody runs inside whom you're trying to apprehend maybe you can use hot pursuit. If the person inside 123 Main Street saw ypu looking through the window at his marijuana plant, grabbed it and made a dash for the garbage disposal, maybe you can use your PC coupled with destruction of evidence and use that exception to the warrant requirement.