If the mouse is in the toilet, it obviously didn't come in through the tank vent or anything else to do with the waste tanks unless there's an opening around the sink drain. A mouse desperate for water can easily crawl in under the toilet seat or lid, and most rodenticides like D-Con, etc., do make mice desperately thirsty which only hastens the effect. The mouse could have made entry in any of a number of ways, so checking for and plugging any openings around pipes, wires, etc., would be a good idea.