RDK,
Do you think your visitor might have been a racoon? Here in the city of Tacoma, racoons have regular trap lines that they haunt during the night because people leave pet food outside for their cats or whatever. I unknowly solved the racoon problem in my yard by having a six foot vynal fence installed around my back yard.
If I was to ever move to a location where vynal fence would be too expensive or out of the question, I would consider building caged aviary type structure over and around my pond. That would actually solve several potential problems. It would keep the leaves out, the racoons out, the herons out, and anything else out that could create a disaster in my pond. The structure could be built to resemble a gazebo (sp) so that it would be more eye appealing. The first one I ever saw had exotic birds and tropical looking plants to give it that minature jungle look.
Happy ponding,
Rich