"If they find something that they can't explain using the lexicon of known methods of design, they don't assume that it was designed."You are partially on to something here, blipey. However, Stonehenge was known to be designed long before it was even remotely discovered how it might have possibly been designed. First the design detection, then the design method detection. The same holds true for ancient tools. "Oh look we have a designed tool" -- design detection based on context, analogy, and function (functional specificity). "Now let's discover a reasonable hypothesis as to how it was designed" -- design method detection. It quite elementary, actually.
As an aside, specification as an indicator of design is also based on context, analogy, and specificity. It is based within a probabilistic context, draws from the fact that intelligence routinely creates specifications, and it incorporates specificity (which includes, but is not limited to, function). Furthermore, there is to date no counter-example of properly calculated specified complexity that is observed to have been caused by a random set of laws (merely chance and law, absent intelligence).
Now, let's just assume you actually knew what you were talking about. Does the reverse of what you assert hold true? If they find something that they *can* explain using the lexicon of known methods of design, do they assume that it *was* designed?
ie: life is based on an information processing system that follows an evolutionary algorithm.
There is much hardware and software design and goal oriented engineering and programming that goes into the creation of information processing systems that can run an evolutionary algorithm.
The application of these engineering and programming principles are KNOWN METHODS OF INTELLIGENT, GOAL ORIENTED DESIGN that are essential in the generation of information processing systems and evolutionary algorithms.
What's more, systems that run off of information and engineering principles harness and control natural law and chance however these very principles themselves are not defined by chance and natural law. Yet, 100% of the time that we are aware of the causal history of these systems, we know that they are the products of previous KNOWN METHODS OF INTELLIGENT, GOAL ORIENTED DESIGN.