But there is another dynamic.
If it were obvious, we would have found them already, and their presence would probably be relatively recent(geologically) in the time record.
The older in time that they were here, the harder it is to find evidence.
So finding evidence in very old strata only means they were here... a very, very long time ago.
But why don't we find something more utilitarian? A crescent wrench designed for a guy with 6 fingers?
Something they cooked their food in? An intergalactic Slurpie cup? Or even a portable toilet?
Something?