We can't detect signals at interplanetary distances? We're still talking to space probes out past the orbit of Pluto. Amateur radio astronomers (note that this has nothing to do with amateur radio) can hear what we think are gigantic thunderstorms on Jupiter, without even a directional antenna. Gain antennas and cold amplifiers can do a lot of very interesting things. A civilization that could get a battleship-sized artifact from another star system into Saturn's orbit could damned well hear us. Interpreting what they heard is a separate challenge, but I don't think it would be rocket science to discern that they weren't listening to thunderstorms.

A peak oil crisis (or a bird flu crisis, or any number of other species of global crisis) might be just the ticket for goading the survivors into getting their lazy asses off the planet before the next crisis hits. The biggest single impediment to establishing space settlements anywhere in our system is our current culture. Peak oil does not mean no oil, and a humbled human civilization could work out ways to get a critical mass of people to Mars with the machinery to keep them alive and reshape the planet to our needs. It wouldn't happen overnight, but there's more to be done there than Kim Stanley Robinson's sleeping pill of a trilogy is willing to admit.

The obvious (and simplest) answer to the Fermi Paradox is that we really are alone, though why that should be is another very interesting and difficult question. I also think that many people underestimate the difficulty of crossing interstellar distances. Given advanced nanotech and an asteroid belt, it would be easier to create a whole new planet (or some sort of exotic habitat smaller than a planet) than go to even a nearby star system looking for a ready-made one.

I wrote an article about this fifteen years ago that may still be kicking around somewhere and could use revising.