Lately there have been quite a few comparisons between SETI and Intelligent Design (ID) or similar creationist ideas. (I will use "ID" as a shortcut for the general idea rather than select a specific brand or proponent of ID or creationism.) Let's consider the idea of sifting DNA or RNA for "intelligent patterns". For example, there may be design patterns or hidden messages left in DNA by alleged creator(s).
Let's not make a distinction between deities, smart aliens, or smart robots being the DNA designers. Even us humans are gaining the ability to manipulate and put messages in DNA, and someday perhaps create life itself. This nullifies the argument that SETI has at least one example of intelligence (Earth life) while ID does not have an example of observed engineered DNA. Both now have humans and human efforts as an example of actual occurrence.
Further, SETI's premise is not any more falsifiable than DNA-ID. SETI not finding anything does not falsify the possibility of there being intelligent life. It only means that nothing was found.
Some SETI experts have complained about this comparison because they claim that SETI uses the nature of the signal rather than the content of the signal to determine intelligence. However, I disagree that the nature of the signal alone is likely to be sufficient enough to prove intelligence. It might find candidate targets to be explored further, but by itself should not be the prime evidence. Identifying candidates is miles apart from proof of intelligence. Candidate hunting is only the first step.
There have been too many false alarms that demonstrate our inability to comprehend the "creativity" of nature. The biggest example was the discovery of pulsars. Pulsars are the collapsed cores of certain kinds of dead stars. They spin on their axis far far faster than regular stars because of the way they collapsed. Some rotate hundreds or thousands of times per second. As they rotate some emit radiation in a very narrow path due to their strong magnetic field. An analogy of this radiation beam is a flashlight laid on end and glued to a fast record player turn-style. (I haven't yet found an analogy for the newer generation who may nave never seen turn-styles.)
From the perspective of somebody in the path of the spinning beam, the signal appears to oscillate on and off in quick pulses. Such an emitted radiation beam was first detected in the mid 1960's. The regularity of the quick pulse made many scientists speculate that they could be signals from an intelligent civilization. The first discovery was temporarily dubbed "LGM-1", where LGM stands for "little green men". Some scientists even considered suppressing the evidence for fear of panic.
Nobody could conceive of something natural that would make such a regularly-timed fast-pulsing signal. The fastest rotating planet in our solar system is Jupiter, which rotates once every 11 hours. Thus, something that rotated hundreds of times per second just did not come to mind. Only after careful study and theorizing was the true and strange nature of these wonderful objects discovered.
A less dramatic example of mistaken identity is the changing patterns on the face of Mars, as viewed through Earth telescopes. The dark markings on Mars were observed to change based on the seasons of Mars (tilt relative to sun). Dark areas would spread during the warmer months and retreat during the colder ones. Frost was ruled out because the equatorial region grew rather warm in the summer and didn't seem to change from the morning to the evening.
One popular explanation was dark plant life spreading its leaves during the warm season and then freezing dead in the Mars winter. We now know it is due to wind patterns and dust storms that sift light and dark dust back and forth based on the seasons.
The lesson is that nature has many bags of tricks to produce phenomena that can strongly resemble life and intelligent life. Nature likes to cry wolf and we have been fooled multiple times.
Thus, if SETI detects a signal that resembles something generated from technology, one should be very careful about concluding it is from intelligent beings. Just because nobody may be able to conceive of a way nature could generate digital (binary) signals, for example, does not mean there is no possible way nature could do it.
A field of different-sized asteroids in front of a pulsar may be able to generate signals that appear to be binary codes, for instance, by periodically eclipsing the radiation beam. I suppose that pattern would be fairly quickly dissected, but there may be other natural ways that look more convincing that we simply cannot think of without actually finding it.
SETI currently uses the "narrowness" of the broadcast frequency to identify candidate signals. Narrowness by itself is not solid proof because there may be undiscovered natural sources of narrow signals. Using narrowness alone would be making the same mistake as the pulsar discoverers.
To avoid these kinds of mistakes, the content of SETI signals should also be taken into account. The fact that pulses or signals are found is not enough; we have to decode or decipher the signals to see what they are carrying. We should expect to find something along the lines of language and/or images before concluding they are from intelligent beings.
If we focus on the content of the signal, then the problem is fairly similar to the issue of finding intelligent signals inside of DNA. DNA can potentially encode anything that we can digitize. What kinds of patterns indicate intelligence? Images or language are probably the most obvious, but not the only possibilities. If a language is too dissimilar to human languages, then it may be hard to detect or hard to decipher. It may not be an easy job and there may never be a final answer to some signals or patterns.
I agree that the probability of finding actual intelligent patterns in DNA is probably quite low, but exploration never makes guarantees. SETI may never find anything either, regardless of whether there are other intelligent beings out there or not. Maybe we really are alone, or maybe other beings don't use technologies we can detect.
Nor may DNA pattern sifting be a very economical use of resources. However, being a poor exploration bet does not by itself make the endeavor non-science. Being inefficient or wasteful and being science are not necessarily related. (Otherwise, we would have to dismiss most government-run programs.)
Unless the probability of finding something is absolutely zero, any exploration is still part of science. Even the most die-hard evolutionists will agree that the probability that ID is true is not absolute zero. Perhaps they will give it a probability estimate resembling one in a trillion, but not zero. No rational thinker would give it a zero.
Intelligent signals from space (for SETI) and intelligent patterns in DNA (for ID) are both testable. If people want to waste time exploring long-shots, that is their prerogative. That does not make them "non-science", but merely a possible bad investment in time and money. Think if them as intense hobbies of exploration. Hobbies don't have to be efficient or good bets. You do them simply because you want to.
A mission to a star system 500 light-years away is currently not feasible, barring a budget-busting emergency. Plus, they may not be there by the time we arrive. Traveling at 10 percent of the speed of light, a typical range for nuclear rockets, it would take at least 5,000 years to reach the destination. The signal could also be coming from a moving space ship rather than a stationary planet. Or the signal could be a one-time burst, similar to the famous "WOW" signal such that we never do pinpoint the source.
In short, finding the source for either sky signals or DNA graffiti is a bonus, not a guarantee. I agree it is probably more likely for SETI than DNA, but probability is not a classification criterion. I have never heard anybody propose a definition of "science" or scientific exploration based on the probability of finding something (other than zero).