As I understand the law, so long as you not driving commercially (Then they can pull you over and search anything they want) your rights work kinda like this:
If a container is locked, probable cause or not, an officer needs a warrant to search it. If you have a locking gas cap, and he wants to search your tank, you say, "Im sorry officer, I got nothing to hide, but I am in a hurry, so by my fourth amendment rights, you will need a warrant to search my tank."
Thats just my understanding, and maybe Im wrong.
That being said, if the officer can justify a probable cause, (Smells drugs, alchohol, ect) then he can search the driving compartment of your vehicle. Most anything he finds is dismissible in court if you can argue the probable cause. Even if the officer thinks he smells drugs, he cannot search a locked glove-box or locked trunk without a warrant.
Just my understanding. I do think I should call my lawyer on this one though.
Swab my exhaust? Thats just silly