Several bodies of known smallpox victims have in fact been exhumed by scientists, but no virus has ever been found. And plenty of the old pesthouse cemeteries have been dug up for water lines, streets, etc. and nobody has ever caught the smallpox in this way.
It survives in scabs for a long time (years), but "indefinitely"? No. People have 'sent up the balloon' from time to time when smallpox scabs were discovered in old doctors' offices, libraries, etc. . . . CDC sends guys in moonsuits and scabs are triple-wrapped and carried to the lab . . . and they never have found any smallpox virus.
There are a LOT of things on the Worry List ahead of a virus that has been eradicated.