I live in a condo, on the top floor. My downstairs neighbor has been complaining of a leak coming from his ceiling, directly below the toilet in my bathroom. According to him, about a cup of water came out of his ceiling this week when I flushed my toilet. I have had a plumber visit twice - once he tried to diagnose the leak from my condo to no avail and then came back to look in the bathroom of the condo downstairs. It is obvious that there is an issue - the ceiling is sagging slightly and there is a crack. There is not, however, any evidence of a leak (no water mark, no brown stains, etc.) and we have been unable to replicate the leak to determine the source.
What to do next?? What else could be causing this issue and what should our next steps be to diagnose??