[D-Coder]

Here's my analysis:

If an alien intelligence has reached Earth, it has technical capabilities at least a century ahead of ours.

Either (a) they do want to be seen by us, or (b) they don't care if we see them, or (c) they do not want to be seen by us.

For case (a), we would have unambiguously seen them by now.

For case (b), we would have unambiguously seen them by now.

For case (c), with their advanced technology, we'd never see them.

So... I very much doubt that an alien intelligence is here.

[kadoban]

This argument isn't _that_ compelling because: send today's tech back a century, use that as your aliens in case 'c'. They would 100% be able to see that tech. They wouldn't know what the hell they're looking at, or be able to do much about it, but they'd see it.

[ZordonShumway]

If we’re talking about aircraft, the combination of modern radar mitigation and modern sensor packages would allow a time traveling plane or drone to be effectively invisible in 1925.

Sure they’re not going to bend light around themselves, but they can fly outside of visual range and 1925 radar technology won’t stand a chance of detecting them.

[viraptor]

It depends where you send it / why. There's lots of places you can send it where there's just nobody to see it. We still occasionally find an uncontacted tribes out there after all, so if someone didn't want to be seen (or even just seen in a place full of cameras), it would be trivial.

[D-Coder]

Sending the tech from 100 years in the future to today is not directly comparable to sending today's tech 100 years back.

By 2125, military aircraft will probably be silent, able to rapidly ascend to 100,000 feet (out of visible sight), and maybe even invisible. So people today, faced with properly-done future technology, can't see it at all.