Because she doesn’t have a history of “missing” before, most likely what has happened is that she was startled when using it, possibly by one of the dogs bounding in for a drink or a snack. You may want to consider moving the box (gradually) to a new location, or getting her a cover for it, so she has some privacy. If you think it’s more marking behavior, there is a great product on the market called Feliway, that once the area she has been peeing in is cleaned up, you spray this stuff and it makes her not feel the need to mark it anymore. Also, you may want to have her checked by your vet, because urinary tract infections and a number of other medical problems can also cause this behavior. Hope this helps. Good luck!
P.S. You may want to call your local Humane Society for options as well, I was on staff at the local shelter for two years, and there were all sorts of remedies that pertain to particular situations. Shelter staff are also willing to try to help you find any solution other than relinquishing her, so check it out, it’s a great resource.