First of all, the ‘visitors’ might’ve in fact be a terrestially derived group who, by dint of having such technology thousands of years ago, have continued to stay technologically well in advance of the rest of us right upto the present day, hence the difference between UFOs then, and UFOs now.

Second of all, if in fact the ‘rocketeers’ of yore were visitors from (an)other world(s), they may well’ve had technology beyond our comprehension even now.

In which case they may’ve left traces of downsized versions of their technical capabilities specially designed to make sense to our species only when we attained those particular levels of technology ourselves.

For instance, as Carl Sagan himself observed, how did the ancient Hindus conceive the preposterous idea thousands of years ago that the universe was eight billion years old, a figure that only began to make sense to Western civilization well into the 20th Century with the advent of the likes of Hubble?

Alternatively, these traces may’ve been left as sophisticated ‘seeds’ designed to sow into our consciousnesses the possibilities open to us.

Then again, such traces may’ve simply been planted to innoculate us against the culture shock that’ll eventually ensue when we finally grow up enough to start hanging out with the big boys, (which sadly seems some way off since we can’t even hang out with each other before resorting to physical and psychological internecine violence!).