A readymade is an ordinary object that has been nominated as an artwork by an artist. Nominating the object as an artwork transforms it into an artwork. This is not a million miles away from The Institutional Theory Of Art, which says that the answer to the question “what is art?” is whatever the artworld says it is.
Nominating a readymade is an act of transubstantiation. Readymades are aethetically null, it is a category error to say “look at how curvy and shiny the porcelain of that urinal is”. You are not looking at the readymade when you say that, you are looking at the non-art object.
Found art is different from readymades, however confused Wikipedia’s entry on the subject may be. Found art is not transformed, its latent aesthetic potential is recognised and capitalised on by the artist. It is a mistake to invoke Duchamp when trying to create a lineage for found art.
Semiotics, the ventriloquial grinding of the symbolism of a cultural artefact to dust, is problematic with regards to artworks. Do you analyse the symbolism of the object qua object, or of the object qua artwork? With regard to readymades it is again a category error; the meaning of the readymade is artistic, not functional.
So in summary:
Readymades are ontologically transubstantiated objects.
It is a category error to attempt semiotics on readymades as aesthetic objects.
Anyone claiming to be both working in the tradition of Duchamp and doing semiotics is confused.