Options include looking for discoloration on surrounding areas and even putting a paper towel or Kleenex in suspect areas in order to detect a small leak that may not have been sufficient to create noticeable discoloration. Depending on the volume, you may be able to hear dripping in order to help trace the leak. And you may wish to reconsider whether the initial diagnosis was correct or whether the repair was sufficient.
Here are three causes that weren't obvious to us that you may wish to consider:
(1) Around the tub spigot: We had a situation in which we had a leak from a bathroom on the ceiling of the dining room beneath it. We re-caulked the tub and shower surround, did a small test, didn't see any water leaking, and thought we'd taken care of the problem. The next time someone showered the leak reappeared. We eventually figured out that the problem was around the spigot: Water from the shower was hitting the spigot, and because there was not a good seal around the spigot, water was running behind the spigot, down to the floor, and eventually into the dining room below.
(2) The actual copper supply line connections--if you're fortunate enough to have access to them without tearing out the wall: In another house, one with only one floor, we thought we had a leak in our shower pan, but during a remodeling discovered that the hot supply line had not been soldered correctly and water had been squirting out between the wall and eventually out through the brick in the adjoining exterior wall.
(3) Diversion due to insulation: We had another leak that we thought was due to a problem with the flashing between the roof and adjoining siding. After tearing up the roof and the ceiling of the room being damaged, we discovered that the problem a poorly-sealed nail in the roof itself. However, this location was several feet away from where water damage showed up. In this case, the insulation directly below the small leak was sufficient to keep the water from running straight down. Instead it wound around and into another room.
The relatively fixed elements are the drain pipe going trough the floor and the vent pipe running to the left. It is complex to change where these exit the room (go into the wall or floor).
As other comments and answers suggest, you can probably shift these pipes toward the back of the vanity. With the vanity removed, you can put in a "sweep elbow" (a shallow curve rather than a sharp 90 degree) from the pipe coming up through the floor toward the back of the cabinet area. You can then turn it upward, again with a sweep elbow, and join the P-trap to that upright pipe.
If the sweep elbow is higher than the floor of the vanity, you can open up the floor to lower it or you can take out a channel in the vanity floor to accomodate. This should still leave enough room higher up to accomodate drawers on one side ot the other.
The vent stack can also be turned toward the back to meet the newly routed drain stack. Elbows in vents are less critical than waste elbows.
Best Answer
Party walls between units must provide a certain sound control. If pipes are placed in the party walls, any noise from the pipes will only have about half the sound control between the pipes and either unit.