Im not an expert, but maybe the oxygen sensors post-cat are going? Since you have almost 50k mi thats not unreasonable. If so this will not hurt your car. Secondary o2 sensors only read catalytic efficiency and are not involved in adjusting AFRs like the primary ones are. If you're not getting any fuel mixture codes then you're okay.
Do you have stock downpipes? Are you running a tune? If you're tuned I would definitely data log and make sure alls well.
Cobb E40 PROtune by Jake@PTF/RR catless DPs/aFe DCI/ETS 5" FMIC/Alpina B3 TCU flash/BMW Perf. Exhaust./BMW Performance Style 313s/M-Tech Rear