[zh3]

There's a simple proof of this; an optimally compressed signal will have all symbols occuring with equal frequency (otherwise you'd be able to compress it more). So yes, an optimally compressed signal is indistinguishable from noise.

However that's the theory; in practice the range of frequencies the signal occurs over and how strong it is at each frequency tells you a lot about whether it's likely to be an information carrier or not.

[jhoechtl]

They send sognals to be heard and discovered. Why would they try hard to disguise if it's meant to be heard by another civilisation?