[rohrmann]

It can be rodents or just about anything like a misplaced screw to wires being mis-routed and pinched or cut. The first direction I would go in would be opening all boxes, with a switch or behind a light fixture to see if a connection came apart that could be shorted. I've had a couple of incidents where the crimp connection was poorly made and just had to be remade, but did not get to the point of being shorted to a ground, but was close. Beyond that, it's a guessing game, but you would need to isolate all the runs between splices to find where it might be if it isn't a bad connection.