It's shocking how many people jump off of the Golden Gate Bridge. I've read that an average of two people jump a month, and those are just the ones witnessed by other people or caught on surveillance video. If someone jumps off at night, their body can be pulled out with the tide and lost in the ocean. If no one witnesses a person jumping at night and their body gets lost in the ocean, they may end up as a missing person forever.
These athletic shoes that are made of synthetic fabrics are preserving feet for abnormally long periods of time. The rest of the body probably gets eaten or decomposes, but the foot is protected and floats back to shore. I don't think that there's anything weird going on regarding all these feet washing ashore, but instead it's the result of synthetic shoes being so commonly worn. Cotton, leather, and other natural fabrics probably break down and don't make it back to the shore.
So I guess my point is that we should consider suicide victims within the last decade. All of the people Hannah1988 listed seem like good possibilities.