The next day I was able to get over there and take a look. I identified the location of the circuit and opened up the light fixtures, outlets and switches that were connected to it. These water-induced shorts tend to clear fairly quickly as things dry up, unless there is a worse problem that had gone undetected. I opened all of the connections and splices in the area that had been soaked, and remade them by trimming back the copper wire that had been saturated. This is very important, because oxidation and corrosion on the copper surface over time can form an insulator that inhibits the electricity from flowing easily between the two wires. It is analogous to if you took the bare copper ends and than put plastic wrap over them, and then twisted them together. The electricity would have a much harder time passing from one piece of copper to the other. In the case of corrosion, it would not be a total insulator that completely blocked the current flow, but it would inhibit it. The electricity would have to "fight through" the "blockage" that corrosion created. It would create resistance, and in overcoming that resistance the wires could overheat and possibly cause a fire. So I make it a practice to remake every splice that had been saturated when I find a situation like this.
Upon taking down the bathroom fixture over the mirror, I discovered an all-too-common practice. The folks who put in the light cut a few corners. They did not bother to install a junction box to support the fixture and contain the live splices as code requires. The just punched a hole in the drywall, fished a piece of BX from the switch box up to that hole, and screwed the fixture mounting strap right onto the face of the drywall. They did not bother to install a connector on the end of the BX. The insulation on the copper wire had been nicked by the sharp edge of the metal shield as a result. This little nick was not enough to short the wires when they were dry, but saturate them in water and guess what happens? A dead short. This installation could not have been any worse, really. There was no grounding to the fixture at all, since there was no metal box to bond it to and no connector to bond the metallic shield of the BX to. It was the classic case of "Out of sight, out of mind."
I was able to clean up the BX and install a proper fitting on it. I cut in a round "pancake box" and attached to the stud in the wall with for solid support. (The paper bag taped to the wall was to catch the plaster dust as it fell, as to not create an awful mess below. I am rather challenged in the IT world and have no idea why these pictures turned sideways when I put them up.) These pancake boxes come in handy in this situation because they are shallow enough to be secured directly to the 2x4 in the wall yet not protrude out past the face of the drywall. That allowed the fixture to be re-installed on a grounded junction box that contained the live wires (they were free floating in the wall cavity before.) Ironically, the 2x4 stud was right next to where the center location over the mirror would have called for, so when I re-did the job the centering of the fixture over the mirror was only off by a small amount. I say ironically because it would have been so easy for the first guy to just mount a proper junction box on the stud before the drywall went up. These kind of things really irritate me. I can't say whether he didn't know any better or didn't care. But the result is the same. So in a way maybe it was good that the flooding occurred and drew attention to this sub-standard work before anyone got hurt.