When we do necropsies at my clinic, we take out all the intestines, heart, lungs, liver, etc. Quite messy, but we explore everything. If her vet did this, it would be very easy to see the twist in the intestines.
That being said, I have a feeling this horse had some issues for a few days and maybe was not noticed. I have had two friends lose horses to twisted guts .One was up in a hunting camp with no vet around and the horse started acting colicky the day before, we walked for hours and when he did poop a bit we hoped it was over. About 6 hours later, he was in horrible pain and since there was no vet around and the horse was throwing himself over backwards from the pain, the owner shot him and we cut him open. It was a twisted gut, from both sides of the twist, the intestines were black and dead and you could see where the rupture occured. We figured, and later talking to the vet was told that when he acted better was when it ruptured, the release from the painful stomach was sudden and fast. BUT, the stomach contents then caused him to become septic and get painful, even worse pain than before and that is when he started throwing himself. This happened in less than 24 hours from beginning to end.
The other was a mare, she started acting colicky and rolling and the owner got her to the vets. We tried everything we could to save her, but she got down, would not get up and started thrashing and died. Because the baby was close to term, the owner opted for us to open the mare up and try to save the baby. This was all done in the horse trailer, we got the baby out and the baby lived for two days, but was just not ready enough. The mare's intestine had twisted and there was nothing to do, she just died.
That is my worst nightmare, a horse twisting a gut and being in agony before dying.