Even if they were transmitting, we wouldn't likely hear them
Just so everybody is on the same page: SETI isn't looking for alien TV, or alien radio stations - we won't hear them. Imagine a transmitter of 1MW were there, transmitting a nice 6kHz wide AM signal. Imagine we used the largest area antenna we have - Mount Arecibo. Yes, the VLA is "larger" in terms of resolution, but doesn't have nearly the collection surface area, and that's what really important here.
Arecibo is 305 meters wide, or a radius of 150 meters (I'm going to keep about 2 significant digits here). So Arecibo would, under the best possible conditions, collect (pi * (150 meters)^2) / (4*pi*(20ly)^2) or about 1.5*10-31 of that 1MW - or about 1.5*10^-25 watts of signal - call it -220dBm of signal. At normal temperatures, the RF noise emitted by the antenna would be about -173dBm/Hz or about -169 dBm for our 6kHz wide AM signal, so the signal would be more than 50 dB into the noise floor.
The ONLY way SETI is going to hear anything is going to be if somebody on the other end was beaming a MASSIVE amount of power very directionally towards our solar system.