It sounds like something from a Stephen King novel.

Some creepy serial killer murdering people and ditching the foot. There's tons of homeless people and unknown prostitutes that go missing every day, plenty we never hear about. Some freak could be sending clues. Since they don't seem to show evidence of fowl play, maybe they are letting the bodies break down naturally somehow and removing the foot? Stranger things have been done by serial killers. Who knows..
Edit:
http://ca.news.yahoo.com/feet-washed...214604738.html
That article, released earlier this year states some victims have been identified. I wonder of its maybe cases of suicide/accidental drownings. Sneakers apparently preserve the foot and do float.