To me seems trying to create a leak is asking for trouble. I see shower tester dams advertised that go over a shower drain so water can build up in a tile shower and flood the shower flood to see if it leaks. If there was no stain on the ceiling when the owner left and there is a stain below the second floor shower when they return you are going to have a problem. My theory is to report what you see and not create problems that were not there when you got there.