Surely the future human being would have ways to detect nuclear waste before digging around

Technology doesn't inevitably progress. Its certainly possible that we lose even relatively simple nuclear detection technology (e.g. a Geiger counter) in the future (or fail to deploy it - I wonder how many construction sites have a radiation check performed?)

And I believe, if we had an idea nuclear waste store, we'd not be able to detect it - i.e. the excess radiation outside of the store would be comparable to natural variation between locations on the earth.