@jamesc -- my mom's case is slightly similar to yours
My mom was diagnosed with Stage IV CRC in 2016 that spread to liver and lung. She did many rounds of chemo to treat the cancer that metastasized - radiotherapy (lung) and ablation (liver), as well as her CRC resection surgery in December 2016. The treatment plan worked, doctors were amazed with her results. She did regular/monthly monitoring and folllow-up CT scans, MRIs, etc. and found no issues. CEA levels were between 5-9 but the doctors considered that normal for her case. Long story short - her life was practically back to normal.

Until her last regular check up this month (Feb 2019) where doctors found a slightly elevated CEA level (12) and a lesion on her spleen and liver. They're doing a biopsy on these lesions to see if cancerous. The doctor said attributing CEA numbers to presence of cancer is misleading.