Right again
The first thing a mothership would do when it got in-system would be to find a local material source and then start mass producing micro-probes and nanobots. The mothership would then transport this new hardware to any in-system Earth-like world and then shower it with these detectors. I have no doubt there are thousands of rusted-out and utterly decomposed nanobots layered into ancient sandstone and coal seams (totally unrecognizable as ET artifacts). Again, the trick is to find the large relics left by ET in deep space. Everything left of the Earth eventually gets decomposed, ends up on the ocean floor and/or subducted into the mantle.
Gary