First off I'm so sorry for your loss.
Years ago we had a 4yo TB who looked to colic. He was treated, thought he recovered and needed to be treated again a few weeks later. After that, he suddenly deteriorated very quickly, we did every bloodtest possible, he was examined by 3 different vets - nobody could decide what was wrong. I went in one morning to find he had rubbed the hair off his head and he fell in his stall and struggled to get back up. Everytime he went down he did more damage to himself so I had to drag him to a paddock so that if he fell again he wouldnt hurt himself. I called the boss, told him to get a vet now, and bring the horsebox. We ended up putting him down in the box so that we could take him to find out what had gone on.
Turned out, while out on his summer holidays from racing, he had ate a a tiny twist tie like one you would see on a loaf of bread, and it had punctured his stomache.