Free: Contests & Raffles.
You can try and get an answer from the WDFW but be careful
I asked the same question a couple years ago here and the consensus was there is no regulation prohibiting it. I wanted a second gun with goose loads so I don't have to do the panic unload, reload when the random goose came over in a couple of the areas I used to hunt.
Just make sure that the unattended shotgun is secured in a fashion that it can't be knocked over (by dog, etc.)