actually, 231 and 232 don't mean "bad O2 sensor" as hinted here, it means that your fuel trim was not within specs and fuel trim can mean many different things and doesn't pinpoint a specific part such as an O2 sensor. A vacuum hose leaking sets off fuel trim codes. If the codes havent returned, they're intermittant and the car would most likely not test bad when looked at via Volvos VST.

If you had a bad O2 sensor on a XC (which there are 2), you'ld get different codes than 231,232.

I'd say that unless the check engine light reappears, keep driving it and don't worry but if it does return, have the dealership check the codes as I understand their position too in that they can't replace parts based on codes that they didn't see. They can't replace parts that test OK (on request) because Volvo tests warranty parts and will bounce their claim if the part tests OK when tested.