I can't commend on Pontiacs, but I can share what I know with mine. OnStar was standard for Impalas in 2004, but mind doesn't have it. I checked, and it was sold under a fleet contract (one of the few LS models for it to happen) to Enterprise as a business car for one of their CEOs (explains the 26k highway miles in its first year and a half before I bought it).

Fleet contract vehicles didn't get OnStar. Maybe a similar deal with yours?