I have an edge 500 and gsc10 speed/cadence setup that ocassionally defaults to 255rpm instead of zero, for unknown reasons. Typically, rescanning for the spd/cad sensor will return to zero, but not always on the first go, and sometimes I've spun the crank too, hoping to wake the gsc10; it's a weird thing, and I don't know why it's happening or what the real fix is, so I've been living with this random problem and awkward kludge for some months. Hopefully someone can supply a better fix for you!
I'll add that I'm up to date on software on the 500, and though I can't recall exactly when the problem started, there have been other problems, like having the unit go into 'hard drive mode' when connected via USB to a computer. Are the problems related? I dunno.