I did some searching and this is a common problem but it seems no one has a straight answer for a solution.. Everything I did read related to a vacuum leak or a IAC.. Another one mentioned O2 sensors not working correctly until they are warmed up. I do know O2 sensors give better accurate readings once up to working temp. Perhaps yous is working enough to not throw a code, but could need a cleaning. I know the O2 behind the cat, if its not warming up is usually a sign of a bad cat, but in this case, I wouldn't say its your 2nd sensor if any. Just throwing it out there.
