One of the first things we tell clients in our criminal defense practice is never give a law enforcement officer permission to search anything. The Constitution places strict limits on when law enforcement can perform a search, and oftentimes officers won't have a warrant or the probable cause they require when they want to search a suspect's property. However, the moment that a suspect consents to a police officer's search, most of that suspect's constitutional protections disappear. For this reason, "consensual encounters" are one of the most popular techniques that officers use to collect evidence. When an officer asks if he can search a vehicle, he usually is not just being polite, but likely has doubts about whether the constitution allows him to perform a search without the suspect's permission.
It is not unusual for law enforcement officers to turn a traffic stop into a full blown search and seizure of the vehicle that has been stopped. How is this accomplished? First, many times the officers, who have made what seems to be a routine traffic stop, know they will find contraband in the vehicle. The traffic stop is used as an excuse to get inside the vehicle, without the police having to reveal the source of their information that the car is carrying. After issuing a traffic citation, a "casual" request to search the vehicle is made. If consent is declined, the officers call back-up to bring a drug sniffing dog. If the dog alerts to the presence of contraband, the vehicle is searched based upon that alert and no one, including the driver ever knows the actual reason for the search.