Christy I am so sorry for your loss, I have two indoor cats myself, and they play with mice that happen to get in the house but don't eat them either.
I found out the hard way not to use mouse/rat poison as a lost 4 ducks that way this summer. I had put some of those blue blocks in the loft of the barn as we have mice and chipmunks running rampant in there. There were no animals in the loft. Somehow the little beggers dropped the poison onto the main floor and the ducks got into it. Not a fun way to die.
Something to think about.. there are ingredients in mouse/rat poison that are found in other chemicals or may just be on your property and you just don't know it yet. It would be worthwhile to take a close look at the property. Also it may clue you in on where it came from if someone did toss it on your property somewhere.