That is a bit bizarre! Although it does look like a fairly old one judging by the design and you are supposed to replace them after eight or so years, due to UV weakening or somesuch, so could be someone antisocial just threw it out there. The sadder possibility is it's a remnant of a crash. Oddly, the photo makes it look like it has no straps - which could confirm the crash possibility, it having been cut off by paramedics? - or were they just tucked inside?