I believe I found the answer. The reason my (very cheap) reader instantly read "0 Codes" was because the seller had temporarily disconnected the battery cable prior to my test drive and me testing the OBD. This resulted in no "ready" systems, so it instantly read 0 codes--because none had the sufficient driving time to accurately interpret the reading.
Unfortunately I found this out after the fact (i.e, after I'd been "taken" by the dealer) when I bought a better OBD reader and saw that the "Cleared" time read 2 1/2 hours--exactly the time I called the dealer telling him I'd be in to take another look at the car. The new read showed that there were not-ready systems (I guess it takes a couple cycles of driving for an hour or so--so the dealer would disconnect the battery before every test drive), but I had the old cheap OBD prior to the purchase.
So--lesson learned--if you're buying a used car an notice the radio has none of the stations programmed--and maybe the clock is showing 12:01 when it's 9 Pm--then it's very likely the battery cable was temporarily disconnected to deliberately clear all systems to the not-ready state so they read no error codes. In my case, I think I've got a bad Cat, so not cheap--but at least the car runs very well and I'll have to live with an engine light until the next emissions test.
My takeaway--spend a few bucks more on a reader that will tell you when it was last cleared, or--if you have a cheapy OBD like I did--if it instantly reads "0 codes", that probably means the systems are in a not-ready state so beware.