JE wrote:

I wonder if this is why if there is intelligent life out there we
cannot trace it. Maybe they all have a phase of going from noisy radio
transmissions to gradually getting more and more efficient transmission
technology and disappearing from view.

As far as I know, SETI are not looking for transmissions leaked from an
Alien civilization. The expected signal from such an event is too weak
to be detected. Instead, SETI could observe signals sent deliberately
towards earth, with frequency near the neutral hydrogen line at 1420 Mhz.

So even if the Aliens haven't done any traditional radio communication
for eons, they could easily build a transmitter and contact us if
they wanted.

The reason SETI hasn't yet detected an artificial signal may be
the low number of stars observed until now, approx. 1000 stars have
been scanned.

Martin.