Why would they remain hidden?
There's more to it.
Wanting to remain hidden is an admission one's presence would affect the observed living forms.
But why would ETI think that? Because it'd fear humans might become a danger ?
If ETI were so many years more advanced, it wouldn't care! Just as we don't care ants notice us or not.
Remaining hidden is an argument used more to demonstrate our importance and influence than to demonstrate they might be here.