One numerical estimation...

Let make only one numerical estimation:

1) T1 = The oxygen, essential life component, in the Earth's
atmosphere exists and therefore emits O2 spectral line at least 200
million years = 200,000,000 x 365 x 24 = 1.75 x 10**12 hours
T2 = Total duration of all METI (Messaging to Extra-Terrestrial Intelligence, http://arxiv.org/abs/physics/0610031) transmissions from the Earth = 37 hours

Thus, L = T1 / T2 = 4.7 x 10**10

2) S1 = Solid angle of the O2 Earth's atmosphere radiation is all
celestial sphere, that is 4*Pi = 12.57 steradians
S2 = Solid angle of METI transmission from the Earth is (wavelength /
dish's diameter)**2 = 10**(-6) steradians

Thus, M = S1 / S2 = 1.2 x 10**7,

and N = L x M = 5.9 x 10**17.

In other words, we can conclude that the probability to detect Earth's
O2 spectral emission by a hostile super-civilization is N = L x M =
5.9 x 10**17 (!!!!) times higher than that of the METI transmissions.

I hope, none of us have doubt about ability of hostile super-
civilization to detect remote planets and make spectroscopy of
atmosphere of these remote planets...

Alexander.