Can you have ulcerative colitis so mild that you dont even know you have it?

Anything possible: Short answer is yes, long answer is if you have family history of IBD (Irritable Bowel Diseases) like UC (ulcerative colitis) or Crohn's, you should definitely see a gastroenterologist as you may need screening studies. Symptoms are usually more evident when the inflammation is bad enough (blood in stool /per rectum + abd pain/discomfort).

Degrees of severity: Yes. UC can go into remission, usually with the aid of medications but sometimes on its own. It is still very important to follow closely with your gastroenterologist even if asymptomatic as flares can still occur and you will need periodic colonoscopy to ensure no polyps or cancer.