Rotary Encoder Schematic Symbol
I am not aware of any standard for this, no. You see various permutations on:


A box with a circle in it and terminals coming out of the side
A circle with terminals, maybe with a rotational arrow in it
A box with multiple switches in it
A box with the words "Rotary Encoder" written in it


The list goes on. So just draw something that says what it is. A rotary encoder isn't really that pictorial a device, hence most things say what it is either within it or next to it.