Hi and welcome to the forum! I am glad that I can help you.
You would like to know why your gallbladder is not visible with an ultrasound and HIDA scan.
Agenesis of the gallbladder is a condition in which the gallbladder fails to develop. Gallbladder agenesis occurs alone in more than 70 percent of the cases. The person with isolated gallbladder agenesis is healthy and no treatment is needed. In 30 percent of the cases the gallbladder agenesis occurs in association with additional malformations such as atresia (failure of opening) of the bile ducts, or normal bile ducts combined with distant abnormalities such as ventricular septal defect, blind rectum with no anus, malrotation of the intestine, and absence of a kidney.
According to the data that you described, you may be experiencing agenesis of the gallbladder. If you don't experience any other issues, the agenesis is isolated. If you experience any kind of symptoms, especially those that might suggest a heart issue, you may consider seeing a cardiologist.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor for further advice and information about agenesis of the gallbladder.