Party pooper
Unless our visitors arrived after the rise of cities, they might have a hard time recognizing intelligence, especially in a population of several million at most, scattered over the planet in small bands. They'd have to "go ashore" and conduct a massive ground survey. My guess is they zoomed in, listened briefly for EM radiation, took some pictures and gathered some specimens, and moved on to the next star. The "sentinel" idea is certainly a viable one, but a thorough search of the solar system for these artifacts would be a lot more expensive than a massive SETI program.
I'm not trying to be a party pooper here, no one wants proof of ETI more than me, I'm just trying to point out how difficult a problem this is.