The sensors are ultrasonic and occasionally do go off when a large or fast vehicle goes by while you're stationary (for example, at a left-hand turn lane). Mostly, they go off from road spray kicked up by semis, but I'm sure that could happen with desert dust too. Very rarely, the rears go off while backing up into exhaust when it's really cold.
Having owned an X5 (and currently also an X6), the LR system is way better than the Bimmers, which false-sound all the time. I swear that sometimes I think the X6 is afraid of its own shadow.

LOL, one time I nudged a half-height parking pole because I would always turn off the PDCs on the BMW...
BTW, only the rear PDC sensors sound off the dedicated sounder - the front PDCs sound using the instrument cluster sounder. This can be confusing because you instantly glance at the instrument cluster to see what's gone wrong - especially because the tone is continuous and longer than when the car's on fire!
