Zero. An object of Voyager's size is much too small to be detected by existing surveys - we can't even track most of passive objects in deep space we ourselves

Zero. An object of Voyager's size is much too small to be detected by existing surveys - we can't even track most of passive objects in deep space we ourselves put there (see for example various hardware left in heliocentric orbits from Apollo missions, or more recently, Musk's Tesla). Even in the unlikely case that it was detected, events would turn out similar to the recent Oumuamua discovery. Some papers would be written on theories of its, most likely natural, origin. A last-second mission to intercept it (much less a sample return) would be considered too costly. There would have to be extraordinary and overwhelming evidence that it's artificial in origin for things to go differently. For a dead probe drifting through space, that evidence just wouldn't be there.