In the case of the Mercedes-Benz, perhaps I have missed some other function of the hubcaps. For example, perhaps they are designed for good looks instead of aerodynamics. In the same way, some authors have made much of the poor design of certain living systems without taking into account their other possible functions in a larger system. For example, peacock tails may make peacocks less efficient, but they have the function of pleasing people. Shade trees convert sunlight less efficiently than algae, but shade trees provide shade for humans, and algae doesn't.
It is possible for a system to have undetected design. If we do not observe the function for which something is designed, then we will not see its functional dependence on anything. A young child looking at a piece of scientific equipment designed to create nanosecond digital pulses may see nothing but a box with blinking lights and not see any function at all. We can therefore talk about "detected design." If we see no design, we cannot prove that it is undesinged, we can only say that we see no evidence of design. With a quantitative measure of design, we may also say that we see only a certain degree of design.
As Augustine of Hippo argued, no thing but God can be perfect in every way. Therefore every created thing has "imperfections" to some degree. We therefore can speak of a hierarchy of design, from inanimate objects to "lower" life forms to "higher" ones, with increasing quantitative measure of design. This is warranted, for example, by the narrative of Genesis 1, which sets mankind over animals, animals over plants, and plants over the rest. Jesus also said, "Are you not much more valuable than they?"
Finding something further down in degree of design does not imply that no thing has design. In the same way, finding a simple little ditty written by Mozart does not mean he was a poor composer. People make various things for various uses, and there is no logical reason why God could not do the same.
We must also distinguish between poor design and systems with good design but which have purposes that we do not like. A shark is a well designed killing machine. This raises the question of the problem of evil, which is a separate question. A well-designed, destructive system does not imply the lack of existence of design. It may imply a well-designed instrument of wrath.
David Snoke, Toward a Quantitative Theory of Design