"Assuming the above is true, then why do we not see any sign of these probes?" Because for example these probes are as tiny as a nano-meter? Hey it is the year 2012 here on earth. In the last decades we have already shrunk computers from the size of a entire room to fit in our pockets and soon our blood. And we ourselves can already build nano-technology, despite not being able to travel around our galaxy. What tells us that intelligent life has to build "probes" so big that we are able to detect them. Maybe these probes are just as large a nanometer and are already flying towards us or were near us or are already on Pluto. Or maybe other life forms shrink themselves to fit into a trillionth of a trillionth of the diameter large spaceship to be able to travel around space faster than light. All of the theories are just expanding from current knowledge as of today, not factoring in any stunning surprises of how the universe might really work :)