Biopsy showed breast cancer cells present in one lymph node, no previous or current bc showing in mammo, us or mri, how is this possible?

LN + for BC cells: All the screening tests to detect breast cancer are not "100%" accurate, even bilateral MRI. Finding a "lump" in one's breast may occur without m'gm/U/S or MRI detection of "possible breast cancer". The good news is that finding these cells when the screening tests were negative, allows early treatment, and a good chance of stopping the disease.