Had ct without contrast and mri of pelvis without contrast due to lower left pelvic pain that radiates down front of thigh to check for diverticulosis/diverticultitis? both showed no evidence of diverticula or diverticulitis.. could it be missed?

Most likely not: Typically diverticulitis and diverticula will be picked up on a CT scan, however there are many other possible causes for pain in the pelvis especially that radiates to the thigh including musculoskeletal and neurologic that would not be picked up by all imaging. Please continue to work this up with your doctor

Not likely: Diverticular disease can be ruled out on above tests and the workup should continue as directed by your doc. Alternate diagnoses will have to be considered.