If they don't use radio technology we'll of course never hear from them - at least not via SETI. So, when it comes to SETI being effective, the interesting case

If they don't use radio technology we'll of course never hear from them - at least not via SETI. So, when it comes to SETI being effective, the interesting case is where the aliens are using radio technology but never advance their modulation technology to the point where the signal is no longer obviously artificial to an observer without knowledge of the coding scheme. It is of course difficult to say whether this is likely to happen; however, it seems contrived for a species to exist that is okay with using math and physics to construct and operate high-power radio transmitters, but refuses (or is unable to) use math to reach the next level of spectral efficiency.