Thanks for the update. I can tell you that my fistula is pretty tiny also. Some times (for reasons unknown to me) it is almost like it has closed up. There is almost no leakage. But other times, my fistula feels the need to remind me that "fistulas never heal, ha ha ha (evil laugh)". It's frustrating when they can't find the tract - they couldn't find mine either. But I didn't have a MRI or anything - only had a liquid dye substance inserted in to my pouch as they waited and hoped to see it come out of my vagina on an x-ray. I didn't care whether they found it or not. It was so obvious to me what was going on down there.
The anti-biotics do in fact help the fistula because the stool is more formed when you are on them and that lessens the chances for leakage. Hopefully you will be able to manage this small inconvenience as I have.
Not ideal, but if your pouch is otherwise healthy than it is a price we have to pay (as if we haven't paid enough already!).