Is memristance missing from our models?
Unlikely.  For there to be a "parasitic" effect, there has to be some physical component of the circuit that, by its nature, is that parasitic effect's cause.  For a parasitic capacitance, all you need is two conductors separated by a dielectric, or in fact even a lone conductor (self-capacitance).  For a parasitic inductor, all one really needs is a length of conductor (self-inductance).  Obviously, these things occur naturally all over any physical circuit you would to construct, and outside the nice "lumped" elements one considers in circuit theory.

On the other hand, it seems that the memristor is a type of device, like a negative capacitor or a functionally dependent negative resistor, that doesn't naturally appear in circuits unless we go out of our way to construct one.  According to the Wikipedia page the idea of memristor wasn't even proposed until 1971, so that such a function even existed was relatively non-obvious, unlike the concepts of resistance, inductance, and capacitance, which were well known effects even in the early 1800s when electrical theory was still in its infancy.