Is there really any benefit to poison? There're variety of effective traps. At least with traps you know where the dead (or live) rat is and it's not going to cause problems like dying behind a wall or something.
BTW some of the older poisons advertised that victims would actively seek to get outside before they died, which would avoid the "died in the wall" problem. Such claims might or might not have been true, but people continue to believe that for all poisons.