I just gave my car a tune up: K&N air filter, ngk coppers, and new fuel filter. I went to reset my ecu and got what appeared to be a ghost code.

I was a code 33, which I know is for an oxygen sensor. Shouldn't my cel have come on for it? It never came on, but when I replaced my engine coolant temp sensor a couple month's ago, I reset the ecu and had no other codes. Is this something should worry about? The previous owner replaced all O2 sensors, but that was some time ago. Any thoughts?

Thanks