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.
There's a leap of cosmic proportions in that assertion.
SETI is looking for a signal that has two attributes: the type is known to be a product of human activity; it has never been observed in the absence of human activity. If such a signal is observed, the first thing that will happen is that efforts will be made to find a natural, non-human cause. Pulsars are a case in point.
ID has no algorithm for determining that a finite string is the result of an evolutionary algorithm or the result of divine intervention or the result of intervention by space aliens. We could detect strings in a genome that are inconsistent with common descent, and I can recall a few years ago, seeing predictions made on these threads that such strings would be found.
In fact, the opposite has been found. Molecular biology has uncovered vast arrays of data consistent with common descent, and nothing indicating intervention to produce a species that could not have evolved via descent with modification. No pig genes in the asparagus.