What are physical scenarious which can be modeled as multiplicative noise?
In communications, a very important multiplicative model is a flat fading channel where the received signal can be modeled as

$$y(t)=h(t)\cdot s(t)+n(t)\tag{1}$$

where $h(t)$ is the time-varying attenuation caused by the channel, and $n(t)$ is additive noise.