Scans missed something?

My wife was diagnosed with stage IV colon cancer. Our current primary concern is with liver metastases. One tumor was found and re-confirmed with a pet scan. It is (was?) considered resectable even though it is uncomfortably close to the blood supply. After 6 weeks of chemotherapy including Avastin, the tumor could not be found in a PET scan . We were initially very pleased. In attempting to confirm if any tumor remained and to get an accurate map of the liver for the surgeon, an MRI was done. It also failed to reveal any tumor remains after review by three separate radiologists. To be very thorough, a 3D color CT scan was done and now a neoplasm is seen in the other lobe of the liver. How is it possible that this object remained invisible through 2 earlier CT scans, an MRI and 2 PET scans? (We do not yet know if it is benign or malignant.) I could also use a resource to help me understand the limitations of the various kinds of imaging systems. Any help will be appreciated and thank you for this website.