One of the most common methods by which officers are able to search a person, their property, or a home, is by obtaining consent. A law enforcement officer can merely ask if the police can search, and if the individual says yes, the search instantly becomes legal.
Often, the individual is not informed of the option to refuse consent, instead asked in a manner such as, "I'm going to look in your trunk, is that okay?" To which the reply is mainly yes. Or, an officer may say, "You can allow me to search now, or I can come back with a warrant," which can be true, although the warrant would have to be written and authorized by a judge who finds probable cause for the reason to search. It's much easier from am officer perspective to obtain consent and bypass going to the judge.
A typical consent search may be as follows: A car is pulled through a DWI checkpoint on a main road, where an officer is standing at the perimeter. The driver, who is not drunk or DWI, is still asked several questions followed by "Do you mind if I search the trunk?" Presumptively, the driver has not committed any moving violations (meaning there is no valid Search Incident to Arrest), and has nothing illegal in plain view inside the car. Something may have caught the officer's attention, however, whatever it was did not give rise to probable cause, otherwise the officer would have been able to search without asking for consent though the automobile exception. Because there exists no valid reason to search under the fourth amendment, the officer was forced to ask for consent. If the driver says no, then the officer should not be able to search.