I think we don’t know of any time travelers in the current day because of two very good possibilities:
- They’re actually here, but they’re deliberately not being reported.
- They are here, but they made themselves unable to be detected by anything in Earth’s technology.
Over the years we’ve considered that solution many times. There’s a problem with the logic.
If we assume that time travel to the past is actually possible and that at some time in the future it is actually invented and we ask ourselves the question, “How long has time travel in this part of the universe been available?” the logical answer is it has been here forever in both the past and future.
If you could somehow stand “outside” the universe as a God-like creature and look inside the universe as an observer you might be able to see the day time travel was first used. It might take a million years but eventually the secret of the technology gets out - there is no way to prevent that from happening. Now the technology itself is present in the past - those people can build time machines. They can purposely or accidentally let the secret out again. Eventually time travel technology is available at all times in the past. This is what our God-like creature sees happening. From that POV the God-like creature can see the technology slowly getting released into the wild both in the future and past relative to the day it was first used.
The view from the inside is very different. To people in a time travel enabled society the technology appears to have always been there - it just is; it was never invented. All these people know is they have time machines and can travel to the past. When they get there they discover the technology is already present. No matter how far into the past or future they travel there are time machines. These people aren’t ignorant therefore they appreciate and recognize the paradox. It’s just a matter of there is no reference point that they can point to and say, “Ah-ha! That’s the day it was first used.” They correctly surmise that time machines were invented and used and the technology ended up being released in the past. People have used it and it continued to be released. What they can’t determine is when it happened because like the Model T Ford, everyone has a time machine.
In this universe there are no MiBS lurking around waiting to assassinate everyone who tries to release the secret of time travel or who is a time traveler. Why would they do that? Time travel is not only not a secret no one would even give it a second thought. The technology is so common that no one would care. They wouldn’t even be looking for more advanced versions of time machines. Someone has already done that and sent the upgrades to the past. The theoretically best possible time travel technology is already there.
This is the point of Robert Heinlein’s short story “All You Zombies”. In that short story the “creation paradox” is even worse. Based on the implied internal logic of the paradox, a time machine is used to deliver the plans to build the time machine that was used to deliver the plans; the protagonist, a man, ends up giving birth to himself, his daughter, his mother, his father and his grandfather who delivers the plans to himself, as the grandson, so he can build the same time machine. Not only is there no creation event for the time machine, there is no creation event for the protagonist. All of the characters are himself.