Yes. We don't have the means to know so. Just like fish in the ocean don't know we exist, but we know they exist.

This can't be answered with any conviction. By that logic, we can be apply this infinitely many times: from the aliens perspective, they are also not aware of other aliens that know about their existence

It's aliens all the way up.

Of course, this 'yes' is a pretty loaded 'yes' even if I agree it's a possibility...

How do you know fish in the ocean don't know we exist?