One of the definitions of ambiguous is 'open to more than one interpretation' (link). Same with its near-synonym equivocal. Some make a fine distinction between polysemy and homonymy. (I think polysemy happens outside of literature.) I have seen the word [i]ambiguous applied to objects, at least in a phenomenological context. There, much can be made of ambiguous drawings, e.g., a Necker cube (link), reversible figure-ground drawings (such as the famous two faces in profile or a goblet one), or impossible objects as illustrated best by works of the Dutch artist Escher.

Ceci n'est pas un seing.