My new Camry XLE has the automatic air conditioning system.  I've been running it in the manual mode and setting the air intake selector to fresh.  Every so often, it switches to recirculate on its own, and then it may switch back to fresh on its own.  Also, I'll turn off the engine for the night while its on fresh, and the next morning when I start the car, it's on recirculate.  I am not using defrost or air conditioning when this happens.

Is this normal behavior?  Does the system switch to recirculate automatically when it senses something, and then back to fresh when whatever it senses is gone?  Or is it perhaps defective?