Ultrasound showed fat brightening on bowel with wall thickness 5.3mm in left flank/iliac fossa yet sigmoidoscopy & biopsies clear? why could this be?

Perhaps the inflammatory bowel disease or Crohn's disease was in a quiescent state at the time of the sigmoidoscopy & biopsy or there was a skip area.. Was a fecal calprotectin done? I believe colonoscopy is still the gold standard for diagnosis of inflammatory bowel disease. Consult a gastroenterologist at a university hospital if you want a second opinion.