I wish this had been a root level comment (so it would be more visible). I think this is by far the most plausible explanation especially if you think about human history and technology differences.

Imagine our current spy agencies with bleeding edge technology spying on the Romans. Its only a two thousand year difference, but there's absolutely no way they could detect our signals and/or equipment (unless we made a serious mistake).

It seems entirely reasonable that another civilization may be tens of thousands of years ahead of us in technological advancement. If you consider the increasing growth rate of technological advancement, its easy to see your explanation.