The galaxy is immense. The space between systems is immense. The space between planets is immense.
We have craft operating in a narrow corridor of space, as they transit somewhere, to a planet or to the edge of our system. They aren't programmed to focus on anything outside of their programming.
We have craft that scanned a narrow band of wavelengths on the EM spectrum, dependent on their mission parameters, or we have craft that scanned a narrow region of space, looking for planets.
We are using communications technology that is based on operating on a planetary scale that has been modified for system scale operations, but in this capacity is limited. We have people who project that this technology will be superceded by a next generation of technology.
With these factors, I doubt that we would be able to detect an alien spacecraft if it was parked near the Moon monitoring us.