Plumbing leak, next step Cutting out walls

[Stephen E.]

There is a leak dripping away from the basement ceiling in one of my units. It is definitely not coming from a pipe in the basement. We were notified about it by the tenant this morning. It appears to be slowly filling a bucket, it is not clear how fast. When they discovered the leak this morning they said a third of the unfinished basement floor was wet. My handyman who is good with plumbing has checked all the obvious spots, sinks, faucets, etc. with no joy in finding the leak. Next he wants to start cutting holes in walls to try and locate the problem. I called a plumber and that is what they said they would do next as well. The drip seems to be in the general area of where the bathroom is but that is two floors up and there are no obvious signs of where a leak might be coming from up there. Does anyone have any bright ideas? I know all maintenance problems can be solved with time and money; it is just a question of how much of both is required in this case. But I would like to minimise the number of holes cut in walls so as to limit damage and expense.