I am fighting a sarcoid now on a 20 year old gelding. It, and about 5 more on the same horse, were dry and flat for the 10 years I have owned and used him. We left them completely alone. We used him for trail riders and lessons.
Last year, for no reason we know of, (no cuts, scratches or ????), the one that was right where his ear and forehead met took off growing and grew into a bloody tumor the size of your fist. The Vet removed it twice but it was VERY aggressive. He cauterized beneath it and injected it the last time but it still came back. We are now putting blood root (an old Indian remedy) on it and it is shrinking, so we are hopeful. Two surgeries were not enough because the ear cartilage under it could not be removed so the sarcoid came back. It is on a really bad place. The other sarcoids on this same horse are still dormant.
I had one horse with sarcoids that stayed dormant her entire life. I had her until she died at 30 and they never bothered her. I still have one of her daughters and she now has one on the side of her neck.