I had a friends pony drop dead in the back of my lorry from a gutteral pouch heamorrage(sp) The vet said he has seen a few of these and the only consistant symptom was a light nosebleed from one nostril - If the owner had bothered to call the vet and have it scoped they could have done something to save the pony.
May not be the same as your horse, i just think it should be checked out.
Mine has had one nose bleed along with a snotty nose. I got the vet out for the snotty nose and he asked me if she had a nose bleed which I said yes. He said as it was only one not to worry to much but if it happens again he would have to investigate it. Touch wood it hasn't.