[lmilcin]

Any civilisation that can detect an object of that size and categorise it as being unnatural in origin would have vastly more advanced technology that would allow them to detect humans on Earth in the first place.

[dylan604]

We dtetected Oumuamua.

[lmilcin]

Oumuamua is a bit larger than a blanket on your couch.

Also, a mirror flying through space is very hard to detect. It reflects very well but only in some directions meaning it is basically black when looking at it from any other direction.

[dylan604]

It's also at about the altitude over your head the premise flew by you.

It was a low brow sci-fi premise where the little probe that could collected all sorts of space dust on the 20 year journey increasing its size.