Abnormal mammogram, but lesion couldn't be found at biopsy. is this unusual?

Can happen. : Keep looking for lesion. May need MRI breast and MRI guided biopsy.

It is unusual,: but it can sometimes happen. Options include more imaging(for example breast mri) to re-evaluate and determine if there actually is a lesion, and re-biopsy. The bottom line is that mammogram abnormality must still be explained, and the first biopsy unfortunately was unable to do that.