Rob Dekker wrote:

There are limitations to technology, as defined by the laws of physics.
Some example was the size of a probe which needs to observe and
communicate. There are many other examples, such as limitation of
any particle or information to the speed of light.

Of course the law of physics will eventually limit any technological
society. But we are nowhere near the limits. Even our microprocessors, where
transistor size can be measured in nanometers, can become much faster.
 Humans are built atom by atom, just think about DNA. If nature can do it,
so can we. When transistors can't become smaller, other technologies will take
over, such as quantum computers.
  Our brain is not very efficient, the speed with which the neurons communicate is
slow compared to electronic communication, so no physical law can prevent us
from building an artificial brain millions of times faster than the human brain.

What would be the security risk ?
If there is no way to find out from the probe as to where it came from, then
that should be enough 'security'. If there is any extra cost involved in
making it 'invisible' or 'stealth', then there better be a good reason for
it. So it would be reasonable to assume that only a portion of interstellar
probes would be 'stealth'. Chances are that IF there is an ETI probe in our
solar system, it would NOT be stealth.

Well, if the probe is captured, maybe it will give away where it
transmits the observed data.

Somebody wrote a book with 100 explanations for the Fermi Paradox.
Personally, I think the reason is that civilisations do not stabilize before
they can reliably travel to other stars (see posting on sci.astro.seti).

Stephen Webb: 50 solutions to the Fermi paradox.

In the end, we still don't know. And, maybe they ARE here. Maybe there is a
probe or two in our solar system, and maybe it is too small for us to detect
(at this point).
We can't even detect asteroids of 10m across until they (almost) hit us.
We have a long way to go.

True, we know very little at this point, but soon we know are lot more.
We can't sit down and think up a solution to Fermi's paradox. We need to observe, gather
data.
In 2007 the Kepler telescope will be launched, and in 2011, after 4 years of observing,
we will actually know how many earth like planets in earth like orbits there exist out there.
That's amazing ! So another parameter in Drake's equation will be known.
  Soon thereafter we will know how many earth like planets have oxygen in their atmosphere, and
probably life, due to another telescope launch. That's one more parameter in Drake's equation.

If our technology keeps up the exponential growth, we will probably know if we are alone in the Galaxy,
within this century. Maybe SETI will detect a signal, as you know a revolution is near for SETI,
the first SETI telescope array is close to being finished. Or maybe one of our future interstellar probes
will find intelligent life.

Martin.