The "code" is not what SETI is looking for. SETI looks for a carrier signal, most likely one with no "code" at all. Such signals are commonly made by humans, but never yet observed in the absence of humans.
Archaeology involves the same kind of search for objects matching ones known to have been produced by humans but not known to be produced in the absence of humans.
Since we have no examples of intelligent designers of life systems with which to compare, we have no living objects known to have been designed.
Some ID proponents argue that we are beginning to design life. Interestingly, the kinds of things we do to increase the yield of crops, or their resistance to disease, are precisely the kinds of things we don't find in the absence of human intervention. The genomes found in living things form a nested hierarchy matching the presumption of common descent.
Your argument that we have no examples of life designed by known intelligent designers to compare with life on earth, begs the question. There are no quasars on earth to examine, yet we have no problem with scientists defining what a quasar is and then identifying such objects in space.
I won't argue that ID doesn't have a long way to go, but to argue that ID by its very nature is unscientific appears to me to be an a priori approach to science.
ID can in principle be validated empirically by finding algorithms that can with a high level of accuracy identify objects that are known to be designed by humans and then using the same algorithms to correctly identify organic and inorganic objects.
We know that such algorithms exist, because we can easily make such distinctions everyday, using algorithms running in our brains.