Can a police officer enter the curtilage of a house in violation of the implied invitation of the homeowner's privacy?

The presence of an officer within the curtilage of a residence does not automatically amount to an unconstitutional invasion of privacy. It must be determined under the facts of each case just how private the particular observation point actually is. Police with legitimate business may enter areas of the curtilage that are impliedly open, such as access routes to the house. In doing so, they are free to keep their eyes open. An officer is permitted the same license to intrude as a reasonably respectful citizen. A substantial and unreasonable departure from such an area, or a particularly intrusive method of viewing, however, will exceed the scope of the implied invitation and intrude upon a constitutionally protected expectation of privacy. What is reasonable cannot be determined by a fixed formula. Rather, it must be based on the facts and circumstances of each case. (People v. Thompson (1990) 221 Cal.App.3d 923, 943.)