u/Realistic-Sale-4471

My father lives alone. Periodically one of his sons will visit, that's the only time the guest full-bath is used. I'm one of the sons, I last visited the first weekend in June. When I went to take a shower (turn on the tub first) some yellow/orange water came out of the faucet before going clear. I was there again this weekend, almost 4 weeks later, and it did it again. He has pex piping, is on municipal water, and the house is maybe 20 years old. Any ideas what this might be?