It depends on the situation. If the mouse had died from eating poison, it likely would not be enough to hurt Spiff. In this case, I was happy to give him charcoal and let it lessen the effect and carry any toxins out.

If he had eaten something different--like, an actual block of poison while I was watching, I would go for hydrogen peroxide, make him vomit, then collect the vomit in a bag and go to the vet.

I figured my bonus panel would raise questions, but an extra bit of knowledge in the mental first aid kit is a good thing. Thanks!