At the end of Main Street, to the left of the castle (facing the castle) near the walkway that heads to Rancho del Zocalo. There is a brownish building...with a door that has a triangle on it, that has a lady and man. That is the secret bathroom.
There's also a very little used one as you come under the railroad, as you enter Main Street. It's on the right side (facing Main Street) and way back behind the "bank". There's a walkway there with drinking fountains, etc. Down that walkway is a bathroom.