How'd he get in there?! Help solve the mystery of the drowned mouse

My current theory is this...

Somehow the rodent got into the sewerage.

Rodent comes up the pipe and into the toilet bowl.

He can't get out of the toilet bowl so swims up the syphon jet.

Once in the internal space around the rim he made his way to the back of the toilet and up through the flapper valve into the tank.

But I don't think he would have been able to just open the flapper valve against the water pressure above. I think he would have had to wait until it was flushed to make his way up. That means our toilet has had a mouse in it for a bit. Ack! We should probably move.