Located on Mare Island, within the Shoreline Heritage Preserve, along Dillabough Rd is the treasure you seek. There is a cement base of some sort in which the cache is hidden somewhere along it's many edges. Please let me know what you think the structure was. Happy caching
Ther is also a letterbox (not the geocaching variety) in this area as well.
Congratulations to TrailerN for the FTF.