security warning in warning center

Just got an 02 Chevy Impala.  Sercurity warning appears in the warning center at times.  I then synchronize the remote unit with the engine off and the warning goes away for couple of hours.
This happened before I put my own alarm system on the car and continues to happen now.
By the way, when I test drove the car it never came on.  Figures!
Any ideas?

also have 02 impala had similar problem,. 02 comes with oe security system (passloc-see owners manual) , and another security system. both can be repaired only  by dealer (to my knowledge), in my case I could remove Key from ignition cylinder while key in on postion, replaced cylinder(not key) also only by dealer.  oe security system will  prevent engine start, but  computer recycles, and engine will start 20 minutes later(some kind of security -eh?) good luck