I happened to catch this evening on the Google Sky view at 9:23 the Andromeda Galaxy ~ 2 1/2 million light years in the background of a candidate star/exoplanet (HAT-P-16) under observation by the ATA "only" 750 light years away in our local Milky Way Galaxy. Might there be beings like us in the Andromeda Galaxy conducting their own search for neighbor civilizations around stars in their galaxy? If so, under our current understanding of the limits of physics (e.g., speed of light, 1/r^2 dissipation of power), they would be effectively isolated from us.
nice image, Rob. yup Andromeda is far, far away and our technology is too puny (and recent) for them to be able to detect us. However, if there are some extreme transmitters in M31 that have been working for > 2.5 megayears, then we might be able to detect them. At least that's what Carl Sagan and Frank Drake reasoned back in 1974 when they used the Arecibo antenna to look at Andromeda and 3 other distant galaxies. If you can look at ~10^12 stars all at once, you might get lucky with the tail of the transmitter distribution. see oserving archive, the reference is Sagan, C., Drake, F., Scientific American, 232, p. 80 (1975). Their sensitivity of 3x10^-25 W/m^2 means an EIRP for the transmitter in Andromeda of > 7x10^25 W or 3.5 x10^12* 2x10^13 W where the last value is the EIRP of the Arecibo Planetary Radar. (check the numbers please, but it's a lot of AO radars!).