How can I locate a mysterious leak in a 3 story rental unit?
you can run a dye trace on the building.  you just buy a uv trace kit and you add the different colour dyes to the suspect areas.  then wait a couple of days and check the area with the uv light.  whatever colour shows up is from the site with the leak.  you may have to repeat a few times to get it to show up.  

as an aside, if its a flat roof, check the adjacent areas on the roof really closely.  its often the case that you will get a hairline crack in a sill or dam and its the cause of the problem.