Is perimeter search without a warrant ever legal?

Question

Are perimeter searches without consent nor warrant ever legal?

What if it's a motor vehicle / car? Is the situation different with a house?

Answer 1

Huge difference between a car and a house. For example, at least in Pennsylvania no warrant is required to search a vehicle on public roads. In other states there are so many easy pretexts that you practically have little protection from a full vehicle search (although the pretext will have to withstand strict scrutiny if evidence found in a search is used to charge you with a crime).

Your house, on the other hand, still enjoys very strong fourth-amendment protections: One of my favorite U.S. Supreme Court cases on the subject is Florida v. Jardines, in which SCOTUS ruled that even approaching the front door with a drug-sniffing dog without a warrant constituted an illegal search. (The majority opinion is worth reading for its illumination of current law on this question.)

Answer 2

Depends on what you mean by "perimeter search". Police can, for example, walk around the perimeter of a car or house if they have permission or the right to walk where they're walking (e.g, in your next-door neighbors' yard) If, while walking, they see evidence in your yard that you committed a crime, they can walk over and seize it (see http://www.olemiss.edu/depts/ncjrl/pdf/Law%20Enforcement%20Materials/Plain%20View%20Doctrine.pdf for more info on the plain view doctrine)

If a K9 is lawfully walking with them, and the dog smells evidence of illegal activity on your property, the police can legally act on the dog's alert.