Have you talked to the upstairs neighbor?
and simply told them you have a leak coming from there bath above you? and asked them if they had or done any thing different at the times when you noticed the leak, (like fill the tub to the over flow (the silver thing on the end of the tub, some times has the a drain lever in it)?
doing a little talking may go a long way,
There is no way any one here can fix it, they can only give you some ideas on how to go about the process of elimination,
And saying it leaked 5,3, 2, ago doesnt mean any except for it is an interment problem, so if the neighbor always takes a bath until the water is running out the over flow ever night, then more than likely it is not the over flow, but if they took a long bath 5, 3, and 2, months ago and they did fill the tub to the over flow, then more than likely it is the overflow on the tub, or some how had a over flow on the toilet, or some other minor disaster that would have allowed water to leak through the floor?
If they never did then more than likely you will have to look else where, and you may have to tear out the ceiling and start to figure what is causing it,
IF it happened after a big rain storm it could be a pipe flashing (on the roof leaking around a pipe,)
All these things have been said before, now you have to do some detective work. or hire some one to do it for you.
and if the upstairs neighbor is not cooperative then you will have to get the condo management involved, (which I would think this would be in there jurisdiction to begin with), and should be notified, as the problem is not actually yours it is the unit above you, your jsut suffering the consequences,
they may even have a handy man or a plumber on call.
even if you do get a plumber in, and he figures it out he will more than likely have to enter the above unit to make the repairs.