"Enclosure" © DigitalBlasphemy.com
Thus, many feel we may fairly confidently look to the electromagnetic spectrum as the most likely means for communicating with most alien species, it may not be the best way to search for indications of intelligent life on other worlds. Most assume that intelligent life will, at some point eventually, if they survive long enough, develop technology that will include some forms of electromagnetic communication / transmission of information which would either "leak" out into space and/or would purposefully used for space communications. These signals would eventually be detectable by sensitive radio telescopes (by those who believe that such intelligent life would use mostly the radio band) or by optical telescopes (for those believe some advance civilizations would use lasers to communicate, at least for communication in outer space).
However, not all species may use the electromagnetic spectrum to communicate with each other on their home planet, and thus not leak out any evidence of their presence or of their advanced civilization into space and, eventually, if we are fortunate enough, be detected here on Earth by our telescopes.
Take, for an extreme example to make my point, a water world where the intelligent species has developed wholly as an underwater organism. Smell, light, and radio waves are not the most effective or efficient means of communication in water, due to the properties of water. But sound is a different matter - sound is a very effective means, which is why whales and dolphins, among others, are able to communicate over long distances (sometimes thousands of miles) via sound alone. Using electricity would be difficult, and so development in that area could be very slow.
As mentioned in a previous post, such a species may also be slow to leave the planet - a creature that needs to be immersed in water has a much heavier load to launch into space than a creature that needs to be immersed in air. Water is dense and very heavy, launching from under water and breaking free from the planet's gravity (especially if the gravity is even heavier than Earth's), carrying enough water to fully immerse one or more creatures, not to mention the mechanisms to not only oxygenate the water and remove waste air (and other wastes) which would be more complicated than those to care for a terrestrial creature, all would add up to a much much more difficult endeavor than it is to launch from the surface, in air, with air breathing terrestrial passengers.
However, it could be argued that such a species would tend, then, to rely wholly on robotic space probes (after they tackle the problem of creating such probes with electricity in a underwater environment from scratch - though one could suppose a heavy reliance on pneumatics, which while adding weight to the craft, would add far less than a water immersed passenger). In which case, they would need a means for communicating with the probes, and thus reveal themselves to other planets.
Although, let us pick, for a another extreme example, an intelligent species that deaf or nearly deaf with rudimentary vision and which communicates mainly by smell and touch. Such a species would not feel the need to develop TV or radio, though they may develop something akin to the teletype where they can, by touch, feel what is being transmitted. Such a device could easily be updated where the information is transmitted through the air via radio waves, which get converted by a braille type mechanism or even a pulsating mechanism which while producing sound as a by-product, it is the variations in the vibrations that can be felt by sensitive appendages which is how the message is deciphered. Thus, such a species would use a means of communicating over distances that would leak out into outer space and which could be eventually detected by radio telescopes on, or orbiting around the Earth.
Finally, someone once mentioned, in a discussion about the Drake Equation, that some species may be paranoid about broadcasting, even inadvertently, their presence to the universe and may restrict/shield and otherwise block signals from leaving their planet - there may be more advanced civilizations out there that we can not detect by our SETI or OSETI telescopes.
Such beings may be paranoid because their physiology causes them to be especially xenophobic, or because of their theology makes them so, or because their ruler is. Or it may be because of a negative experience they had with another alien species. Or it just may be they logically thought about it and saw how violent their own people can be toward each other over differences and how more technologically advanced civilizations tend to easily conquer less advanced civilizations and figure they should play it safe (there are those here on Earth that present this very argument against our having SETI programs which purposefully try to make contact with other worlds).
We've had many examples on Earth of cultures closing their borders to avoid contact from other countries because of the paranoia of the leaders - mostly to limit the influence of "subversive" ideas from other cultures that may cause the people to reject the leader, or the paranoia of a people in reaction to a negative experience with outsiders. And so it could also happen with at least some other alien worlds.
"Optical SETI - OSETI." Life in the Universe. 8 August 2001. 13 January 2008. <http://www.lifeinuniverse.org/noflash/OpticalSETI-07-01-03.html>
Rockefeller University. "Two 'Noses' Are Necessary For Flies To Navigate Well." ScienceDaily. 29 December 2007. 13 January 2008. http://www.sciencedaily.com/releases/2007/12/071226230117.htm>