A loud splash

Last night I was sitting at the computer with our dog laying in my lap. It was about 6:00 pm and had already grown dark.We hear a loud noise like something being dropped into relatively deep water. Both I and my dog heard it. She reacted, jumped up and started sniffing about the Library, I had the impression it was something metal. The thing is we don't have any standing water in the library. The nearest source of water would be the bathroom. The bathroom has a water source, of course, but not any standing water. There is the jon (commode). The water in the jon isn't deep enough to make that sound. Just another unexplained noise in an old house.