There has to be a pump somewhere near by with an access panel. If it is not in a closet on either side of the bathroom walls then it most likely was buried.
A slow leak is the hardest to find. water dripping from the light is generally a sign of much water leaking.
Start a series of test with help from your neighbor. First get a large bucket or two and fill them with water from the sink or kitchen - not the shower and start dumping them down the tub. If you have drips below you know that your P trap is leaking and that can be fix with ease from your neighbors suite (he/she will not like this).
If this produces no leaks you have ruled out the waste line. Next run the tub filler not the shower. This can isolated a faulty tub filler and may be an easy fix.
Next run the shower with a piece of hose so that the water doesn't hit the tub deck or any tile.
If no leaks you are down to 2 options. Leaking pump or jet or water making it's way in through the cracks.
You can buy/rent a small camera and drill a hole from the other side and try and see under the tub.
This will get you started.
You need your neighbors help.