How likely is an adrenal tumour not found on CT scan or X-ray? My bloodwork indicates possible adrenal tumour and MRI is next step. Thks!

See below. An adrenal tumor would almost never be visible on an X-ray . Most adrenal nodules are easily visualized on CT scans (anything larger than a few mm) but may not be adequately characterized. Dedicated MRI of the adrenals provides the most information. Also, MRI doesn't use ionizing radiation (like X-ray and CT do).
Not easily seen. An adrenal tumor can be small and is not easily seen on CT scans, especially if done without contrast or if thin cuts at the level of the adrenal glands were not done.