While SETI is busy searching for signals from alien civilizations, there are scientists who think we can find proof of advanced extraterrestrial life much closer to home - namely in our DNA!
Instead of leaving artefacts for humans to find once they are sufficiently evolved, an advanced extraterrestrial civilizations might instead incorporate information into the human genome, allowing it to be copied and maintained over immense periods of time.
The downside with leaving behind alien artefacts is that they will not survive for millions of years. A coded message hidden in our DNA, on the other hand can be saved for a very long time.
The coded message would only be discovered once the human race had the technology to read and understand it.