Sorry about the lack of updates, but this damn drain pipe had been leaking since day one. Luckily, day one was a little over a month ago. Still, I had to take apart a cabinet to get to these photos. As you can see, we also had a mold problem. The mold is now clean, but the drain pipe still needs to be fixed.
That one wooden wall, yeah, the one with the water stain on the bottom, is holding up a granite counter top. Thus, it cannot be removed. This make is quite difficult for one to work on said pipe. Looks like I need to cut a hole in the back wall of the cabinet, repair the leak and reassemble the cabinet. Damn it!