Since the private individual, say an employee of the company, conducts a search for evidence on property that is owned by the company and is not acting as an agent of the government, a warrantless search is permitted. The Fourth Amendment does not apply. For review, the Fourth Amendment guarantees: The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized. The exigent circumstances doctrine provides an exception to these guarantees if destruction of evidence is imminent. Then, a warrantless search and seizure of evidence can be conducted if there is probable cause to suspect criminal activity. The other answers describe exceptions where the private individual is subject to the Fourth Amendment guarantees.

Passwords can be compromised and must be protected. In the ideal case, a password should only be used once. The changing of passwords can also fall between these two extremes. Passwords can be required to change monthly, quarterly, or at other intervals, depending on the criticality of the information needing protection and the password's frequency of use. Obviously, the more times a password is used, the more chance there is of it being compromised. It is recommended to use a passphrase instead of a password. A passphrase is more resistant to attacks. The passphrase is converted into a virtual password by the system. Often time the passphrase will exceed the maximum length supported by the system and it must be trucated into a Virtual Password.
Reference(s) used for this question: http://www.itl.nist.gov/fipspubs/fip112htm and KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 36 & 37

Access control is a bit like the four legs of a chair. Each of the legs must be equal or else an imbalance will be created. If you have very strict Physical Access controls but very poor Logical Access Controls then you may not succeed in securing your environment.