This happened to us. About ten years ago, two year old DS had been poorly, no fever, but hadn't eaten very much for a couple of days.
He was just getting better when it happened as well.
Scared the holy crap out of us.
The hospital kept him in and the nuro ran a test with a skull cap thing on his head to check his brainwaves or similar. There was nothing wrong.
The only test that came back out of whack was the Ketosis one. To this day, despite never having had a real diagosis from any of his peds even after further tests, we have to make sure that he doesn't go long between eating something. If he runs around a lot, gets excited, is stressed...more regular snacks are required or he faints.
He has never had another fit (thank god, becuase it goes down as one of the scariest things that has ever happened) but we have never satisfactorily resolved what the underlying issue is either.
Which means I am not a lot of help to you really. Except to say, I get it. I know how scary it is and how unsettlng not to have an answer and to have it at the back of your mind that it might happen again.
So big fat hug and I hope they offer you further testing soon, just to rule stuff out so a little peace of mind is on the table.