The Great Pyramid and Stonehenge were built by the same people. The Sphinx is much older but wasn’t so much built as hammered out of an outcrop. Anyway, none of this stuff is all that old. 30,000 BC is old.
If we cared we could probably detect their former presence by evaluating the percentage of rubidnium present in the sand used to build the dykes in Nederland.
This will certainly happen to Chicago at some time in the next 35000 years. Archaeologists will then postulate a predecessor civilization that sent missionaries South to build the ruins of the complex on top the hills at New Orleans.