What about radioactive waste?
Plutonium has a half life of 80 million years, and although it is found sparingly in nature, a massive clott from a war, accident or commercial venture would look suspicious.
Oh, and there's three hundred (useful) isotopes discovered so far that can only be made in a reactor, that should not be present near dinosaur fossils.
It's also possible that seemingly naturally found radioactive elements, do need to be made in a reactor too, but they are just leftover remnants of the Voth culture and wwejust figured wrong that they are naturally forming.
Which means that our leap into atomic technology was a million times easier than the Voth's who had a smaller table of elements to work with before they started smashing atoms.