In normal warrants, law-enforcement has to show up, knock down the door, and then they can collect evidence that has to do with the warrant that they have in their hand. If they got the key to get into the location, then they only got access to one location. No one else magically gets access to the house or other houses just because the police got in.
That's not true. If the cops have a warrant (actually even if they don't but they Are preventing a criminal offence... e.g. if there are blood curdling screams coming from a building, or smoke that smells like hard drugs is billowing out of a house) they can kick the door down and bash their way in without requesting permission.
Happens all the time. A dude who has just killed somebody can't just refuse to answer the door and be protected because nobody's shown him a warrant.